I paid $60 for my bike used off Craigslist. After pedaling it for awhile, in went a $160 Grubee GT5 from BikeBerry. Rode it like that for a year.
Then I decided to go 4-stroke and enter the 2010 EZM bike contest. So I devised a plan, made a partslist, and tore apart my perfectly working, perfectly running $220 motorized bicycle.
Went from this:
To this, $1200 later:
Have another $500-$600 or so in it since then. It was well worth it to me, too.