Let's assume it takes 1750 lessons to trans a skill and 700 for miniskills.
Generally, to be able to become a fighter worth a damn, you will want to trans all three of your guildskills, plus Survival. If you want to stretch it a little further, you'll want Antidotes, and maybe some investment in Tattoos, Weaponry and Perception (let's say Expert and assume 433 lessons to reach Expert) for their nifty benefits.
Some professions (knights, mages, rangers/amazons) may get away with just transing two guildskills, and you don't REALLY need every single thing stated above, so your mileage may vary on exactly what is needed/desired. This is just an estimate of the high-end costs.
Three guildskills: 5250
Tertiary skills: 1300
So, using very loose estimates, it takes about 9000 lessons to become an entry-level combatant (i.e. you are not flailing about and dying horribly due to lack of skills learned.)
A fresh noob at level 1 has 150 lessons. You get 5 lessons per level gained after that, and 100 bound credits from 10 to 70.
At level 80, you will have 1145 lessons to spend. This means that you'll need to obtain 7855 lessons through credits.
One credit can be converted into 8.5 lessons (assuming no prior conversion) and bound credits into 6 lessons. This means you will need 1310 bound credits or 924 whole credits.
Current credit prices on the market are 8,000-10,000 per, and it seems like you'd reasonably be able to get them in bulk for that much (or potentially less.) You will spend 7.4 million gold at the least, and 9.3 at the most. Or, you can shell out $300 for 1000 credits via the website.
For $25 a month, you can get Iron Elite, and obtain 1260+(30m) bound credits worth of lessons per month (5 lessons a day, 100 bound credits a month, 5 bound credits extra for each month after that, 60 credits for selling two tokens). Using this method it'd take you 6 months to get the lessons you need at a total cost of $150, plus you'll have a pretty nifty XP bonus to go along with it.
So, if you're looking to be a badass, grab an Iron Elite membership and play half a year. It is statistically worth it!