I'm surprised that nobody has mentioned this before but it appears that id designed the game to make purchasing ammo, grenades, health packs, wingsticks, and ingredients totally unnecessary. You can find everything you need to complete the game without buying anything.
On my first play through I felt it was all too easy to play the card game and earn as much money as I wanted to then buy all the overpowered ammo types, sentries, grenades etc. That made the game simply too easy. So on my second play through I set myself the following rules.
1. Don't buy any ammo, grenades, health packs, wingsticks, or ingredients.
2. I can buy schematics, armor upgrades, and anything to do with the vehicles. (This is because these things cannot be scavenged, only bought)
3. Don't use health packs ever! (Health packs make the game stupidly easy)
4. Never die (or you must restart the entire game). Basically hardcoe mode with this rule.
It is not only possible to complete the game this way but It turned out that id set the game up to allow this type of play. It's very obvious by what ammo can be found in what situation that id thought it out and set the game up to work just like all their old games. In previous id games you found just enough of each ammo type to do the job, but you couldn't blindly use the same gun to play through the whole game. Indeed, Rage is set up to give you just the right ammount of all the basic ammo to use the most appropriate weapon in each mission.
It also limits your access to all the overpowered ammo types like pop rockets, explosive darts, regular rockets, sentries etc. You will need to save these ammo types to use on the more difficult enemies but id set it up so that you have enough of them at just the right times as long as you don't waste them at the wrong times.
This style of play makes for a much more thoughtful game. At the start of each mission I found myself looking through my weapons and ammo to plan what I would use. I was more focussed on accuracy because every bullet counts. I couldn't blindly craft things because all those ingredients were limited to what I could find and I wanted to save those pop rockets for the Gearhead Vault later etc.
It also forces you to use the more basic ammo types. Regular pistol ammo becomes valuable, especially nearer the end of the game. Basic assault rifle ammo is your mainstay as feltrite ammo is hard to come by. Having to use the basic ammo means the whole game is more difficult. You can't simply mow your way through everything in double quick time and you have to be careful not to get mobbed. Grenades are also in short supply and need to be used wisely. Overall it makes the game more difficult and richer because of the thought required in what ammo type to use at what point, and the requirement to be accurate with every shot. Having said that, I will reiterate that the game is beautifully set up with just enough of each ammo type to get the job done. It's so clear id planned it that way. You will see what I mean when you play it. Like you will find more pop rockets at the start of the Gearhead Vault, and more explosive bolts at the start of the Jackals.
Adding the third and fourth rules, to never use health packs or die, will obviously change the style of play for some. I'm already the type of player who does everything possible not to die. To that end in my two playthroughs I haven't died once. It calls for a more thoughtful approach to the game but it's a style of play that I find more immersive. Battles need to be taken more seriously with no room for risk taking. Planning how you will clear a room full of enemies with minimal use of ammo, and only basic ammo types, is far more interesting than charging in and hitting everything with pop rockets.
Oh, and I nearly forgot to mention that the game is also perfectly balanced as far as money is concerned to allow this type of play. You always have just enough money to buy that armor upgrade, or schematic that you need. Just enough to buy some more rockets for your buggy etc. I also had a rule, that I haven't mentioned, to not play any of the minigames that allow us to make money. They are also unnecessary. There is just enough loot to sell to then buy the things you need.
So in conclusion, if you found the game too easy then play it like any other old school id game and you will have a much richer experience. The ability to buy ammo, and ingredients is there to allow us all to play the game however we want to, but for the more experienced fps players I strongly suggest that you don't.
You could also try rule 5: Don't use the defibrilator.
I started out with that rule. I intended to play through the whole game without ever falling down (except the parts where the game does it as part of the story). Unfortunately I sat down to play my final session and I was at the start of the Jackal territory. I got charged by Jackals, as we do, and because I was at the start of a new session I stumbled over my keys. Instead of changing to shotgun, I hit the crossbow button again and it changed ammo types instead (I play on PC with weapon hotkeys). Fumbling, I hit the same key again and changed ammo types again instead of selecting the shotty. Argg. That was my demise. I ended up using both defib charges at that point because on the first defib I hit the mouse button instead of E so I stood up with basically no health again. I finally got my shotgun out and dealt with the situation without dying so not all was lost. I'll have to try again to complete the game without using the defibrilator. Lesson learned, I usually take a couple of minutes to cycle through my hotkeys at the start of a new session to make sure everything is instinctive and I didn't on that session. Stupid.
Hopefully others will take up this challenge and leave their comments to inspire others to try it. To me, it's the way the game was meant to be played.
