I'd have to check the specifics here Frank, but off the top of my head the LiPo we acquired last year was a 5Ah 14.8V. It weighed in around 400-500g, around $300AUD shipped.
I don't really know where those 5Ah/7.4V figures came from, but they're not correct. I think you'll find the packs Bryan is talking about are (and I'll take a stab at the numbers here) 4S2P. That's 4 cells in series and 2 lots of them in parallel, which gives 14.8V under load and a total capacity of 2.5Ah. Then, if you take 2 of those packs and put them in parallel, you stay at 14.8V but now have 5Ah of capacity.
On paper, the max currents (ours was 20C=100A max, higher pulse/burst) could easily start an FSAE car and based on the assumptions Bryan has made, (5Ah, 50A, 90% available capacity), I get a cranking time of 324 seconds.
We tried to get a lithium battery going last year, but ultimately, we weren't confident enough with the charging/balancing circuits so we kept it off the car. It can and does work though. Did you see UWAM's lithium battery?
Lithium batteries aren't for everyone though - you'll need to have an elec eng develop a charging/balancing/monitoring system and babysit it, but for under $200 you can lose 2.5kgs and have something nifty to talk about at design.
Oh, and there is also the minor detail of the pretty violent explosions if you get it wrong.
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