Some colleagues were staying in Panorama Plaza, just 5 minutes from the airport and next to the Spice, a big mall away from the city centre. Some others and me personally next to the centre, prices between 450-750 I think for your needs, 20 minutes from RIX.
Regarding lifestyle I suggest you to take a look and a week in Riga checking the surroundings, a lot of people like it others not at all...