£300 -500 sounds a bit over the top. What is causing these high prices, is the growing popularity of e bikes which use the same 48v 20amh configuration.
Have you considered re-building your battery pack with new cells. It would cost nowhere near the cost of replacing the whole pack. I used to build battery packs using new cells and shrink tube, these were for model race cars, it wasn't that difficult, but I was using only 4 or 6 cells.
Just had a look on eBay and found these batteries. Reading the description, it seems that this UK based business, builds there own battery packs to exactly the spec that you require .. might be worth contacting them to save yourself a small fortune ?
https://www.ebay.co.uk/itm/Rechargeable-Battery-Lii-50A-26650-5000mAh-Li-ion-3-7v-Powerful-Torch-Ebike-UK-/124206844896?_trksid=p2349624.m46890.l49292