It depends on many things, e.g.
1) How does it compare with your current position?;
2) Are you coming on single status, married staus, etc.
3) Break the QR20'000 up, e.g.
* Housing - accommodation in Doha is quite expensive, but it also depends on what conditions you are prepared to live in... (see clasified section). Finding anything in Doha under QR10'000 is difficult (again, depending on what conditions you're prepared to accept).
* You'll probably need between QR1000 - QR2000 pm to purchase a vehicle, depending on your needs.
4) Have they offered you a settling allowance? - you'll need it to cover many expenses.
I see you're currently in Dubai & I must say I've compared both & found the cost of living to be very similar (water & electricity is in my opinion more affordable in Qatar).
Good luck with your decision....