E-Maxx cells

Hello to everyone, it is my first time
I am interesting to buy a Traxxas E-Maxx 3905 and I would like to ask about decent batteries packs. I am not looking for something extreme or sophisticated. Of course I would like to exploit the most of the 16.8 V motors, so I am looking for 7-pack cells, but I was concerning about the autonomy and I don’t know how much mAh should I look for. If it has autonomy for about 20-22 min it would be brilliant.

I did a little search and I came up with this: http://www.promatchracing.com/proddetail.php?prod=42007
(Product ID: 42007 - ENER-G 4200 7 CELL FLAT STICK PACK)

Will these cells meet my needs for 20 min autonomy?
Why are they cheaper than the others? (Usually I found cells with the same characteristics at about 45-55 $)
What is your opinion?
Thanks in advance