There is no point in spending more money on preservative, than the raised beds cost in the first place. Lesson learnt.. B&Q were clearing out fence and shed paint last week, it was £2.00 for about 10 litres. That would do until you need to replace it. One thing that will add to the life of the wood is, after you have painted it, get for some polythene and staple it onto the inside of your frame, just down from the top edge. Allowing it to lay on the ground, make sure you leave only a couple of extra inches to encourage water away from the frame. This will slow the rot down. It will also stop any transfer across to the plants. If you are happy with the idea of raised beds, you could always replace the boards with decking boards when the time comes. Mine are 10 years old this year and are still sat on the allotment unpainted.