if one of your sills have collapsed, which is what it sounds like, or have you jacked it up on the chassis leg?

if the sills collasped u need to get the car checked out, cars always look solid until u take the underseal off of paint so u can see the steel.

cost wise im only guessing here if its isolated corrosion maybe £100-£150 based on local garage rates for welding. if its a new sill could be talking a bit more.

when was it mot tested last?