As a long time user of Apple Mac computers, I have been subject to Apple's offers of ridiculously price memory upgrades. I can remember the time when their memory was so expensive it would add several hundreds of pounds to an order. I must say though, that I have never ever ordered memory from Apple, as there is a much better alternative.

Crucial Technology offer memory for all types of devices, including Apple computers. They offer a lifetime guarantee too and guarantee compatibility. Their website is very well laid out. Determining what memory you need can be done in two ways. You can either select your computer manufacturer from the drop down list, then the computer type, then the model version... you are then presented with the options for different memory upgrades.
The other method is to download the small Scan My System utility, which when run, looks at your system and tells you what type of memory you need.

Pricing is really great, in some cases less than half the price from Apple. PC memory is also very keenly priced. Delivery on all my orders has been quick. Plus they have an 0800 number, should you need to call them. I give them top marks.