Replacing the subfloor in an airstream is so common, mostly because of the propensity of plywood to rot, but there is an entire subforum on this site dealing with floor repair and replacement.
You might wanna spend a little bit of time browsing that sub forum, or simply searching for “rotten floor “to see a number of threads of people who have tackled this very issue
It is extremely common, perhaps the most annoying and common problems inherent in airstream
The cost of doing so well vary depending on whether or not you do it yourself or hire out to have it done and all the materials that you used to replace the floor. If you go with plywood, which is the cheapest material, even today with the inflated cost of everything during Covid, or if you go with a composite material like Coosa board which will last forever. Replacing it with plywood will last for about 10 years at which point you have to do this all over again when that plywood rights away. If you were planning on keeping your trailer for longer than 10 years, you might want to explore other options.