Not exactly your issue, but similar, and links to old thread may help.
https://community.ebay.com/t5/Technical-Issues/Anyone-else-having-problems-when-listing-BIN-price-se...Try changing something else (like a shipping option) first, then change price, then change shipping option back.
It's sounding very much like an old issue from years ago where one of the listing form scripts that watches numbers entered in the price field (to make sure a BIN price is 30% more than an auction price, or that an entered price is a valid number, etc) is borked. Used to be that changing some other info in another entry field first prevented that.
If you want, post the item number of the item and we can look to see if anything is externally obvious.
