A regular deep cycle battery and 400w inverter are going to run $100-$150. They're useful for other things too, I still have mine and used it to power my laptop and DSL modem in our ice storm a week ago. That battery is 5 years old now and still has pretty good power, not as good as new but I don't need it for that anymore. If I'd been more careful with it it'd probably have lasted better too.
I forget the exact % you can discharge one and not damage it, somewhere around 50% I think. The battery has 110ah, the heater draws ~34a, lets say another amp for the inverter for 35a. I can then run the heater for about an hour and a half bringing my battery to 50% capacity at which point the car should start in any but the worst conditions. My car would start at -20F after and hour and a half on the heater with no drama.
If I were going to do it again I'd wire the battery into the charging system and put a bigger alternator in the car (bunch of posts on that recently) then wire the inverter directly to the block heater (hardwire the extension cord I mean) and then put a remote car starter between the battery and the inverter so I could trigger the thing from inside the house/apartment/office.
Another side effect is you've got that big heavy battery in the wheel well where the weight can do you some good in snowy conditions. If you went all out with 2 batteries you'd have 3 hours of heat at 50% capacity and balance the weight.
I'm all for Optimas and originally planned to buy one for this project but they were about 2x the price at the time and I couldn't see enough advantage in them and their reduced capacity (the yellow top was ~87ah IIRC) to make it work for this project. Of course the big advantage would be a MUCH lighter battery to hump into the house...
