We mounted a radar on a transom post last year, and while I had a friend that was able to do some welding, the total cost of SS pipe and fittings was around $150.
The trick is getting the radar high enough above the deck to increase range and avoid microwaving people on board. Ours is 7 feet above the cockpit seats.
We used a 2" SS thickwall tube and fashioned a bracket to the toerail to anchor the bottom, and welded it to the top rail of the stern pulpit. The radome is a very small one, 12" and only about 7 lbs, so our needs are smaller than yours. Even so there is some motion in a seaway but it seems OK. Though we planned to gimbal it, in the end we did not. So far we have been motoring when we needed it, and were in relatively sheltered water.
You might look around for a broken mast section from which you could make the post, and then come up with a way to support it. The aluminum section would be stiffer than the SS tube we used, but it's a bulkier installation.
There are backstay brackets available commercially, but they are indeed pricey ( near 2000 euros on one site I found)