I've just clicked on some of my favourite links to see how easy it is to visit Sardinia cheaply. Needahotel.com is a great place for finding cheap accommodation but their main site isn't particularly user friendly. A better way to access their system is to go to Ryanair's website and click the 'Cheap Hotels' link. (You don't have to be a Ryanair customer):
http://www.ryanair.com/site/EN/
I entered some random dates in June and came up with a 3-star hotel in Alghero for just under �30 per night. (That's for a single room. Twin accommodation might be even cheaper, per person).
Ryanair fly from Stansted to Alghero. Their summer schedule hasn't been uploaded to their website yet, so I couldn't check fares. I've previously thought about flying to Alghero, however, and I think that it wasn't too difficult to find flights for �30 or less each way. (I actually tried some random dates for a week's stay in January and was quoted �1.29 out and �0.01 back!).
There are bus connections to the centre of Alghero from Ryanair's flights but a taxi should be quite cheap as it's only about 10km.
If you don't live near Stansted, ThomsonFly have services (from about �30 each way) from Birmingham, Gatwick and Southampton:
http://www.thomsonfly.com/en/index.html
For tourist information check:
http://sardegna.indettaglio.it/eng/
Hoping that this helps,
Chris