I'd say 600 hours quoted above as being a slight exaggeration. At least in my experience - on the 73 fleet my average was around 700 - which when you consider for the winter period I averaged 4/5 flights (even as little as 1 flight) per month, it gives a taste of how busy the summer can be.
the figures quoted above in regards to FOs earning potential within TUI can also be true, but is by no means guaranteed. It has nothing to do with the crewing levels being wrong though. As a charter airline, it has to be adaptable, and therefore crewing at around 80% and offering considerable remuneration to pilots willing to work on days off, is actually cheaper in the long run than having a surplus of pilots and paying for pensions, national insurance, etc for the next however many years. Most duties attract 2 WDO payments, some even 3 - so its quickly quite easy to see how 90k is achievable if 1. The opportunity to work on days off is there, and 2. You are willing to be flexible.