I don't mind paying mercs initially, but not every time I take them on as a follower, so is there any way to get around having the repay a mercenary follower upon rehire? If there's no way to directly get rid of the repayment option then I was wondering, I remember when hiring Faendal I could access the money I gave him as payment for training (which I didn't take advantage of). Can I do the same with mercs so I can retrieve the money I gave him/her first time round?

Many of the hirelings will follow you again for free if you rehire them again within a certain timeframe. Not sure what the timeframe is though.

I don't think a hirelings' gold will show in their inventory when you open it to trade inventory with them, but then again I've never tried it. It might if you use a mod like UFO that lets you see ALL of a follower's inventory, instead of just the stuff that's usually up for grabs. Or it might be that money removed from your purse when you hire them disappears into the ether, much like the Jagged Crown does when you turn it in. I dunno.

I know with Jenassa (in the Drunken Huntsman in Whiterun) you have like three days to hire her back to get her services for free. After that, it's another 500 gold.

I'm not sure if each merc follower has a different expiration timeframe or not though.