2005 Volvo S40 Gear Selector Service

Transmission problem
2005 Volvo S40 Front Wheel Drive Automatic 67000 miles

when selecting gear the reverse works then when selecting drive it gets stuck on neutral. Then about 5 minutes pass then you can select drive. They say it can be a sensor or a relay.I havent taken the car to a dealer yet, Im afraid it can be something major because I dont have the money right now to fix it or get a diagnostic check up.I will soon be able too but it will be great if I had a heads up first on what im dealing wit and a estimate on how much could it cost.