Typical riding pattern with moderate assist is 15 watt hours per mile, lower if you pedal harder. Times your range - defines watt-hour size of battery (how much energy it has).
Typical pack is 48V 15 Ahr = around 700 watt hours - divided by 15 will give you a range of maybe fifty miles. A Ping Pack or Headway pack of that size will cost you around six hundred dollars, and will last many years, due to the robust LithiumIronPhosphate chemistry - the safest type - recommended for beginners. I run Pings and Headways, both good solutions. The Headway packs seem to have more reliable BMS systems - I've had several Ping BMS's pack it in and had to replace them. Two packs in parallel will double your range. Lighter packs with different chemistries can be found but are typically less safe (LIPO), or have less longevity (LIMN).