The compensation doesn't have a formula, but takes into account a number of factors: age, years at club, first team appearances, offer on table from existing club and substantiated interest from other clubs. This only applies if an agreement can't be reached between clubs.
There is a development fee that uses a time based formula and effectively gives a minimum for the compensation. For Cadden, for example, this would be about £180k.
Somebody earlier extrapolated from Telfer and Fergusson tribunals based on Cadden's length of service at well. I don't think it's that simple and age may count against Motherwell here as he can walk for nothing next season.