I guess my point was that simply putting a motor on a bicycle and going 30mph IS abusing it, bikes weren't designed to hold up to this....
As for the cost of the bike, a new bike, I feel yah... I paid $545 for the mountain bike I'm using, in 1988! (my first ever credit card purchase! LOL) It's built like a tank, good vintage stuff. With that in mind I would suggest you keep an eye out for rummage sales, want ads, maybe local search on ebay, even on garbage day.... for a vintage mountain bike you could get for next to nothing and put a few bucks and little elbow grease into so the cost is lower and the quality higher. Often times people toss great bikes because the gears dont work right, he!!, you dont even need the gears with a mb! (even though you could probably fix them). Just something to keep in mind...