Have you considered putting slicks on your mountain bike and playing with the fit to get a bit more aero ride? Could be easy and cheap and give you something workable until you find the right used road bike.

I have a 1992 Trek 8000 that has Onza bar ends on it that allow me to stretch out a bit and I run Specialized Hemisphere tires that are a good mix for street and light trail use.

I use it to ride around with the kids, pull a trailer etc, but I have done tons of 20-30 mile road rides on it.



You can get bar ends that let you strecht up and out or drop bar ends for around $20
http://www.amazon.com/s/?ie=UTF8&key...2Ck%3Abar+ends

A good set of tires can set you back $50 - $75 but you can find city tires for a mtb for as low as $10 each
http://www.nashbar.com/bikes/Product...54_-1___202470

Just a thought, that for under $100 you might be able to turn what you have into what you want or need.