If you are building a Gaming PC, buy from decent Hardcore Gamer's brands like CORSAIR, OCZ. KINGSTON, GEIL as they all have good selections. Usually they average of prices from around £50-100+.
For normal stable, but still of decent brand, the likes of CRUCIAL or SAMSUNG. SAMSUNG being my personal favourite of all since they happen be the best in memory making. With low prices right now, 512MB would at least be around £25 average.
I would highly suggest that when you do get good RAM, at least 1GB of RAM is highly recommended. Go for DDR3200 i.e. 400Mhz speeds as standard.