My input, I will try to be as unbiased as I can since I don't agree with some of the comments posted here already.

You simply cannot compare an E60 M5 to an E39 M5, for starters the E60 is a V10 and has way higher running costs than an E39 which is a V8. I have long been an M enthusiast so like for like the reliability of the E60 vs the E39 is something I feel I can comment on with a high degree of confidence. In general you will find both models have bullet proof engines, however the E60 suffers form a multitude of transmission related issues and the E39 doesn't. The transmission of the E60 is a far more complex beast and therefore brigns with it more potential issues, especially if you're looking at a cheap early model, thats as simply as I can put it.

Don't go comparing an expensive/low miles example E39 to a shit example E60, it's like comparing a mint 4x4 saph to a rotten 3 dr.

E39 in general have not depreciated by any significant amont in the last 3 years, I'd expect to reasonably pay £8-9k for a decent one, there are obviously more expensive examples out there, sadly most of these are not value for money and that's where you'll take a hit. Finding a low miles E39 is pretty hard, so some do command a hefty price, but comparing that to an old sierra then it's perfectly justifyable.

I paid £8500 for mine back in 2010, I'd expect to see £6.5-7k for it today, that's not lot of money to lose on a car. M5 is easily the best car I have ever driven in terms of reliability, performance, luxury and build quality. If you want a more "reputable" source to back this up, ask Stu @ MSD for his opinion. Good luck with searching for the right car.