Visas on arrival cost Qatari Riyals $100, which is about $28. The exchange rate is fixed to USD. You can only pay by credit card - no cash. The arrivals terminal is a completely separate building from transfers and departures. Buses meeting QR flights stop at both arrivals and the departures terminals due to the heavy connecting travel, but terminating flights (ie. BA) only stop at the arrivals terminal. To get from one building to the other, disembark your original bus at arrivals, wait outside the arrivals terminal for a QR flight bus to pull up, and get on and go to the transfers terminal. Should only be a matter of minutes