The Dewalt batteries are actually A123 cells which are the best on the market for lithium. They aren't cheap at all by any means. They are actually the most expensive batteries you can buy. Lithium is measured by the Watt-hour or WH. Those dewalt 36v packs are 36v 2.5AH packs so you get about 36 x 2.5 = 90 wh which will give you about 3 or 4 miles range before it cuts out. So if you bought two of them, and you say they are about $150, that's $300 for about 6 - 8 mile range. That's horrible. If you can find new dewalt packs on ebay for $50, that'd be a pretty good buy. Whenever you look at lithium, measure how many watt-hours it has in the battery and that will tell you whether it's a good buy. My 36v20AH thundersky was $360 + $30 shipping so $390. I got 36 x 20 = 720 wh so 390/720 = about 55 cents per watt - hour.

I don't recommend SLA unless you think you just want to enjoy the e-bike for the summer and then it's not too bad. I think you should keep looking for a couple more weeks until your motor comes. Get a 2nd opinion on the pingbatteries. I personally don't think they are all that great but you are welcome to ask around and see what people think. Try endless-sphere.com forum . Someone just posted a ping problem today actually.
http://endless-sphere.com/forums/vie...hp?f=3&t=19682