As far as I heard about Qatargas, I don't think they pay such a less amount for the position you mentioned. Perhaps it is a private company holding a contract with Qatargas. As the work is in Qatargas site, they call themselves Qatargas. First of all you have to clear this matter.

About accepting the offer, it depends on your situation back home. If you are jobless or find it hard to get a job there, you can think over accepting it. But Leaving a well established job for this amount is nothing but foolishness.

Present day Qatar is very expensive please note.

Wish you all the very best.