On Sunday, January 4, 2015 at 10:06:51 PM UTC-8, Davej wrote:
Is this worth doing, and if so then how?

There is a lot more to a soil test than ph.
If your soil is deficient in a element, you need to know which one it is to avoid the shotgun approach of using "tons" or everything hoping you will catch the missing deficiency.
This is particularly important when you buy soil or planter mix to use in raised beds. You really have no idea what you are buying in many cases. Also seasonally, the outfits that mix "garden soil" have different sources of materials and one load may be great and the next load pretty crappy.
A lot will depend on whether you mix in compost to your garden also. No compost, soil chemistry more problematic. Soil chemistry also more difficult to control, keep balanced, in pots.
Last time I checked in So. Calif and outfit 30 miles away charged $65.00 for a full test. They were in Temecula and accepted samples for testing through the mail.
Hopes this helps.
Jim in So Calif