Batteries for 12- to 24-V motors
I'm just going to say that 9V batteries are a bad idea, they generally have a very small amount of power stored in them. You're better off going with a bunch of AA batteries than with 9V batteries, if you're looking to get the best ratio. 9V are only good for the cases where you need the higher voltage of 9V, but have very low current requirements. A small SLA (sealed lead acid) battery is a good choice, I've seen these used before on medium sized robots. They are good for when you need a lot of power to drive motors and such, are very cheap compared to NiMh or LiPo rechargeable batteries, and you don't mind the extra weight. Make sure it's 'sealed', they cost a tiny bit more, and have slightly worse performance, but they are safer and don't require any maintenance.

If space &amp; weight are at a premium, and you don't mind spending lots of $$$, then you can upgrade to LiPo (Lithium Polymer). For example, you could get four of these 3.6V 6Ah LiPo batteries, equivilent of a 14.8V 6Ah battery, and it would weigh less than 1lb, whereas a 12V 7.2Ah SLA battery would weigh around 6lbs. Of course, the LiPo's will run you about $80 versus $20 for the SLA. So as always, there's a tradeoff.

Also, your numbers above are wrong...a standard (huge) car battery is closer to 50 Ah than 5 Ah. But those weigh like 40 lbs, versus the smaller 6lb one I linked above.