My conversion has some pretty heavy cost constraints as well. I used Sealed Glass mat Lead acid batteries so I don't have to worry about spills or topping up the water. They are pretty heavy, but will work fine for me. From the site I posted, I ordered a kit that contained my Motor, Controller, Contactor and throttle. I spent a touch more to get the controller with regen braking. After I had all the stuff, it was just a matter of hacking off stuff I didn't need and mounting the new components.
I just did my power circuit test this weekend. It's pretty exciting to see your motor turn for the first time.
I used...
Motor: Etek-R (brushed) rated 48Volts
Batteries: 3 - 12V Lead Acid Batteries (36V system)
Controller: Sevcon Millipak 4Q (has regen breaking)
Contactor: White Rogers 24VDC 600Amp peak (Also used a big fuse)
Throttle: Magura twistgrip potentiometer
Others: odds and ends such as welding cable, wires, connectors, bolts, sprockets, chain, etc.
Everything here was under $1250.00 after shipping, but that didn't include the batteries or sprockets.
My calculations show it should go about 35mph and have about a 20 mile range (stop and go).
I'm using 12 teeth on my drive sprocket and 72 teeth on my rear sprocket.
I hope this helps!