Its worth a shot, but I'd assume the reason they charge so much for extra RAM is because they have a corner on the market, and therefore can get away with it. Since you don't have access to the box, and are most likely going to need more ram, they can get away with charging so much for it.
Much like a crack dealer giving you the first little bit on the cheap.
Your also asking them to let you introduce an unknown quantity into their systems. How do they know you haven't done some sort of crazy NSA hacking on those ram chips so you can break into their network. (Ok, thats pretty far fetched, but I hope you see what I'm trying to say).
In response to the other answer
"You'd be doing them a favor to let them know a valid concern you have as a customer. You'd be finding out if they value their customers. Go for it."
Thats about the same as suggesting I go complain to the oil companies because of the price of gas. Their making tons of profit, raking in the dough by charging the customer for what they can't do themselves (pump and refine oil into gas). OF COURSE they know their screwing you, and their laughing all the way to the bank about it.
The server hosting company in question is doing something you can't, host a server in a big fat pipe. I'm sure you could go lease an OC3 and host one yourself if you want 5 more gigs of ram in your machine...but 5 more gigs is what, 125$ a month...vs how much is an OC3 lease? Back to my example of the oil companies: You could go buy an oil derrick and a refinery, but are you really going to save any money for just hosting one machine? Nope. ANd thats where this (and every other) hosting company has you over a barrel.
Its worth a shot, but I'm sure they know what their doing when it comes to their pricing model.