http://www.greenhead-holidays.co.uk/
This is our regular place for when we ride Dalbeatie, Maibe, Kirroughtree etc.
Sauna, pool table, decent showers. Very cheap (although we normally go in Autumn/spring) and the owners are friendly. The only downside is that there's no riding from the door, however it's only 10 minutes drive from Dumfries and the Pizza/Fish and chip shop near the river in the town will deliver if you ask nicely.
[edit] Just re-read your post – it's not near the sea as it's 6-7 miles north of Dumfries. It also sleeps 6 so might be a bit big if there's only 2 of you going (although in the off season it used to be £60/night which is ridiculously good value)