I have the 4000mAh MaxAmps saddle pack for my 501X and have been completely satisfied! Sure, it's not legal for BRCA, IFMAR, etc. sanctioned racing (even though mine do have the $4.99 USD optional hard skin wrapped into the pack), but for club racing, they have been great!
Looks like Bulldawg R/C here in the USA has the TrakPower 3200 saddles for $75 USD! What a bargain!
http://www.bulldawgrc.com/trli32lisapa.html