that's not a bad price, since the bike's complete. as far as motorizing it, it's not in super great shape, so i'd go for it.
here's the problems with it though;
the wheels will need to be upgraded for sure. rusty 15g spokes won't hold up. so either new wheels for 100 or so, or re lace those oneswith bigger spokes. to do that, you'll need to drill the hubs and rims. if you can't do it yourself, it'll cost more than new.
if you get new rims, you'll need a new chain and front sprocket, 'cause the skiptooth won't work with a new rear sprocket. you might need new cranks, too. i had an old 38 hawthorne and the cranks weren't wide enough, even to bend them out a little. also, Hawthorne's used a weird bottom bracket thread, so it's chain, cranks, chainwheel, bottom bracket...
and those pedals suck when they get old.
needs new tires and probably tubes.
and you gotta make sure the motor will fit.
you're looking at $2-300 in upgrades, not including the motor.
it gets kinda spendy.
i've been on the look-out for basic platforms to build old bikes, like just the frame, fork, fenders, y'know, just the basics to still call it "vintage."