My car fits the freeway capable and on-budget criteria, but not the range.
Ben Nelson's 1996 Geo Metro
In the 144v configuration:
Geo Metro - Free, after selling parts.
Open Revolt controller, built from parts - $300
Used Charger - $200
Forklift motor and new brushes - $100
Used batteries - $144
Machinist work (adapter plate/coupler) $300
Misc - cable lugs, used springs, etc. - about $300
The whole project was under $1500. I typically run the car on a 72V configuration. When I do that, it's not a freeway speed car, but looks very stock. Plenty of room in the trunk for groceries, instead of filling that with batteries.
The achilies heal of the project is that the used batteries DO NOT perform the way new ones would.
The combination of "Freeway Speed AND 50 mile range" is what I hear people ask about most ofter. That's just a tall order for an inexpensive, homebuilt EV.
My current plan is using the EV for ALL SHORT TRIPS, and a diesel/bio-diesel/veggie oil vehicle for longer trips.