Rental is expensive in Qatar. Set aside at least 6000 for a decent accommodation. You'll probably need to travel to work everyday & I assumed it's your own expense. Put that at 2000 per month. Food - 1500. Internet & Phone Bills - 1000.

If you're taking 20,000 that would leave you with roughly 9000 per month. I haven't include going out, personal expenses, etc... Let's add 1000 assuming you don't drink & don't go out much. So you'll be saving 8000/month, roughly. If that is enough for you then yeah take the 20000 salary.

Just a thing to remind you, I don't think house rents will be going down any soon. Chances are they'll increase the rent the next time you're renewing your contract.