I can only answer from my own personal experience, and this is a bit out of date now...
You pay £23,000 to easy. This can come from your own loan, the savings under your bed, or wherever.
Your salary is reduced by £5,000 pa for 5 years. Note that anything that is calculated on your salary is also reduced accordingly (pension, share issues, bonuses and so on). You are
not immune from the further salary reductions that easy make (10% for six months after joining/command).
On top of your salary, easy pay you an untaxed £430. This you then send off to your lender, plus the extra because £430 is now not enough.
When I did the initial TR, I did the JOC too. There may be a long wait (unpaid) between the JOC and the TR, so plan financially for this. Furthermore, you are unpaid (and not even an employee) for the duration of the TR course, until you pass the LST. Then you discover that pay is one month in arrears! In the end I went 4 months between pay checks.
Be mentally prepared to do the TR alongside other FOs who are paid and who are on expenses. We had situations where TRSS were not allowed to get into employees taxis when going to the same sim.
All in all it cost me about £30,000 in fees, expenses and lost wages. This excludes my relocation costs - I was verbally "promised" a base in England, but during the course I got my letter of base allocation and I had to move abroad (at my expense, during the TR course). In 2003 it was the only way to get a jet job in Europe. The situation now is totally different and I would advise long and careful thought before committing to this.