They need your bank account details to pillage it. It costs so much and takes so long because allegedly they have to check the mod is safe. I think this is cobblers. More like it is to put you off attempting it in case the lawyers come back at them if it goes pear shaped. (informed legal opinions are most welcome!)
I tried to get a sensible, stc'd and safety related mod done some years ago when under the rule of the CAA. The whole beaurocracy was so obstructive, I eventually just gave up.
A great aid to flight safety I think not.