That's a ridiculous price.
You can run an aftermarket clutch no problem with a stock transmission. You just have be aware that you now have a clutch that will take a lot more abuse than will the stock clutch. If you drive in a reasonable manner, it won't make a difference. But if you slip the clutch, then, yes, your transmission is going to complain.
Until I put in my new gears, I ran a ClutchMasters 300 Kevlar clutch. The engine was putting out about 300whp/270wtrq. No problem. The stock gears were in excellent condition when we cracked the trans. Only problem was a couple of bearings that had been damaged because of the fluid that had been in the trans.
The clutch cost about $725, and the only thing that had to be done additional was to turn the flywheel (yes, the stock flywheel is just fine; make sure you turn it, though). I think the cost to turn the flywheel was about $45. A good mechanic will probably charge about $450 to put in the new clutch, maybe less. It took us just a little over an hour to pull the trans (using a hoist and air wrenches). Took about 2.5 hours to put the clutch in and replace the trans (we weren't hurrying).
Hope this helps.