Most cost effective way to build raised beds.

tyres, painted white, other tyres stacked in the middle, or a range of different heights, you could use each tyre to grow a herb or something...you could use the really big ones from tractors etc...

sorry, I've got tyres on the brain.

Not the exact size you mention, but bath tubs picked up via freecycle or local builders would cost you nothing. You could then collect wood too and hide the sides of the baths. Wooden pallets are 3'x3' and are free, could be used as retaining walls, with extra bits of wood filling the gaps.

Miffy,

easiest material to work with, easy on the eye, and and also fine for perching your bum on would be untreated oak railway sleepers. You can get these in 8ft lengths so quite practical to cut one in two for a couple of ends. Nail through some verticle rails internally at the corners to fix to each sleeper on the length and bredth and then you just have to fill them. That will take a fair bit of soil. If expense is an issue, start with a couple of layers per bed and then add another couple of layers when you have funds available.

I used pressure treated tanalised (tanalith - e or something if I recall correctly) wood (having done a lot of reading online to convince myself it was suitable for veg growing). I got it from a yard near where my parents live but you can get similar at B&Q for what appears to be quite a lot more money... but I used 47mm thick wood that was about 150mm wide if I remember correctly and bought it in 2.5m lengths as that was the best fit to the size of beds I was making.

3-4 "heights" of it would bring you up to seat position I guess, depending how far you sank it into the ground (if at all?), I only went in an inch or two to stop soil escaping out the bottom.

Its a bit narrow for sitting, but you could use any offcuts you have, cut to fit in triangles shapes and put in the corners, for little seats at each corner of the beds? it would slightly reduce your growing area though.

I fixed them together with decking screws into 47mm square posts I made out of offcuts, which only went about 15cm into our combination of incredibly compacted soil and rubble below the beds, they are full of soil though so are unlikely to go walkies...

You would need some decent tools though as it would take you months with a hand saw with the wood being that thick.

I have built 8 raised beds in the last year. All 4ft x 10ft. I used 3x3 fenceposts for the corners, and 6x1/2 for the sides. It was all tanalised timber but I lined them with DPC to try and keep the moisture in the soil and off the wood. All in the timber, DPC, post tops and nails cost about £70, I think.

I'm planning more shortly, as I'm building up a new garden from scratch and the raised beds are forming the design.

I got some old scaffolding planks from a scaffolding rental place for £4 per plank (13 feet long) a quick sand then painted with with fence paint on the outside. It costs a lot to fill with soil so someone on here reccommended the lasagne method (layering up compostables with paper and card) which has worked pretty well.

Cost about £50 for 3 x 13 foot long, 2 foot wide beds

i edged our lottie beds last weekend
was going to use ordinary 5x1 but lengths needed would have been £120,,,no way
visited a local fence panel making company to us and they plank and re-saw there own logs and the outter edges lick a ( shape @ 6ft long and 10 inches wide were 30p each
maybe an idea see if theres anything like that round you

Don't know if you're near the coast, but a lot of wood gets washed up round my way. I made a big raised bed from timber that I found on the marshes in the creeks and seawall.

used scaffold planks are cheap too, ask at your local scaffold depot