Back when I was compulsively playing GH1, I found skill trainers immensely useful. That said, I think the system is poorly designed from a game-balance/gameplay perspective.
I am not sure whether it is the exact formula, but rule of thumb I worked with was: [dollars] = [XP required to level skill] times [one hundred]. You just 'buy' skill XP, which is otherwise acquirable by actually using that skill (although that quickly becomes intractable as you go past level 6 for most skills). The other, usual, way of leveling up ofc is to spend character XP on a skill.
So generally I estimated the amount of skill XP I needed, based on the current skill level, then pay for about half or quarter that amount several times (based on simple formula above). This is generally also useful since you lose control of your character for quite a while of game time if you are training high-level skills, and your PC may go hungry in the process and may need to take a break before training further.
edit: As it turns out, skill xp does not carry over. I dumped 6.25M cash on level 2 cybertech skill. My knowledge is 24 and I had maximum possible reaction for the professor. End result? Level 3 cybertech skill. Whereas paying her 50k 4 times netted me level 6 in cybertech.
That is sad. I honestly thought skill XP would carry over, inferring from a few other things in how GH1 works. But hey, by the time I can truly afford trainers, I usually don't mind inefficiently burning tons of cash to get muh skillz.