GamersGate are currently offering seminal space strategy games Master Of Orion 1 and 2 for less than the price of a bus ticket, which is pretty good value in our book. These two titles are widely regarded as the best in their genre and well worth a play despite their slightly dated visuals.
My only reservation is that the complete edition (including Master Of Orion III, which isn't anywhere near as good, admittedly) frequently gets discounted to around the £3-£5 level by digital retailers. Your call.
Impulse Driven are currently charging £3.26 for both Master Of Orion, Master Of Orion II and Master Of Orion III, which is a decent saving for two and a half great games. The first two games are absolutely exceptional 4X space sims with an incredible level of depth and diplomacy, but it's worth noting that the third outing is significantly less impressive. Still, you're getting a lot of game for your money - and be aware that Impulse are known for snatching their deals away at fairly arbitrary times.