In my opinion:
"An assassin will utilise any and all means necessary to do the job that they are tasked with, practicality is what keeps them alive, not a sense of honour or dignity for their victims"
On a more realistic level, invisibility is an advanced illusion spell. It's loud and costs a sh*tload of magic. I find it well balanced because you need many other skills and perks in order for it to work effectively, hell, you can even hear your own footsteps and whenever you activate an object it wears off.
In short, no, it's not "cheap"
If you're an assassin, you've already invested an intelligent level of points into Stealth perks. What else is an assassin char going to invest in? Thus, you are already dead silent by the time you've built Illusion up to where you can cast invisibility. And after all, being one to 'utilise any and all means necessary to do your assassin job', you've also built your magicka up to around 200 points, so that you can easily make a steady stream of invisibility casts, as needed. Sneak in close, cast invis, kill, cast invis again, go hide, magicka quickly refills, rinse, repeat ad infinitum. I know, I made my char that way, it was completely easy to manage, all along the way. Unless you deliberately decide not to take any perks in Stealth, and leave your magicka at default bare bones level, it truly is cheap to cast, resource-wise.
And as long as you don't stand up while running and invisible, nobody is going to hear you (especially since, of course, being a brilliant assassin, you're also wearing the DB boots that completely silence you), The times I've dinked around with it, there was never any kind of resource issue, always plenty of magicka when needed. Even with default magicka, that would probably still be the case, since all you really ever need is two casts- one to get in, another to escape (if that). The actual time you need to be invisible, to do a successful assassination, is rather short. Most of the time, you are just stealthing, not invis'ing. And when you cast invis from stealth, nobody sees or hears you do it, since of course, you also got the 'Silent Casting' perk much lower down in the Illusion tree.
If you've done your build intelligently, casting invisibility is completely silent, costs a reasonable and sustainable amount of magicka, and only takes the expected amount of perks that any stealthy assassin specializing in Invisibility would normally make.
In short, if that is the build you were deliberately making anyway, and if you did it the right way, yes, it's pretty cheap.