Javier, the only "paying" you could justify ( & that is really at a push) to yourself ( & finally, you are the ONLY one you have to justify it to, even if many of us here are outraged/sanctimonious) is if you join Ryanair or some other company, who specify. . . pay for the type/reach the standard required/ here is the contract (even if it is "only" as a contractor in the 1st instance.)
Anything else is a waste of time.
Do you realise how many guys are available at this moment to prospective employers ,proud of their 300/500 hrs on a 737/A320 ?. . . it is, really, a VERY big number, & you will NOT be employed any time soon with that experience level as long as a company can instead take a Cadet & earn even more off his back.
Cadet versus you with 300hr on type ? same story, in terms of ability, but the former earns them some more money. . . do not be Blind !
Do not pay for anything without an offer of employment, you are supposed to be intelligent with some power of reasoning. . demonstrate this please.