I understand there is already a shortage of Sr90 and/or other suitable issotopes, so the price will surely be higher if we start building lots of these cars. I also suspect the amount of Sr90 you suggest produces 20 kw or less, so you also need a super capacitor or a Tesla type battery to climb steep hills at 70 MPH or accellerate to 60 MPH in less than ten seconds. On the plus side your nuclear car can supply all or most of your homes energy needs when it is parked in your garage.
Unprocessed high level nuclear waste might substitue for Sr90 at very low cost, but the radiation hazard is higher, even if the containment does not fail. Neil