There is a guy that posts on the ampedbikes forum that travels 21 miles each way. He uses heavy SLA in a Pannier Pack so the weight is low. I think he is using 12AH batteries at a cost of about $100.
Another route is Ebay Lifepo4 which are dropping rapidly. If I were you I would buy a 48V 15AH Lifepo4 pack for the amped kit getting you around 25-26 mph and around 30ish miles. The packs are lightweight and you can charge at work right out of a backpack (thats what I do)
Their speed control and motor are rated for 60V+. I rode mine at 48V for 3 months on a lifepo4 pack then burned up the bms trying to connect it wrong with no fuse.
With Lifepo4 you will get upwards of 2,000 cycles. Even after 2,000 cycles it only drops to about 85% so for around $400 you can get a 3-4 year pack.