I did mine, so it was almost zero cost, just my time/effort (I won't count the Aldi welder, a nice little £60 unit, perfect for this)
A premade kit from Froggatts is £66 + postage, but I didn't really want to do a replacement mount, just a repair plate / patch.
It was still pretty awkward, with very restricted access underneath. I also had to lift the carpet and remove some bitumen-looking soundproofing material, for safety
Working below in the very tight space to remove underseal for the welding was messy and more difficult than the actual welding... As with many jobs, preparation was the bulk of the work.
I'm sure we'd all be interested to learn how much the welder charges you, I suspect a few £hundred would be the minimum.
No doubt he will find the best / quickest way to do the job - Full respect to mobile welders and mechanics - sometimes it's best to leave certain jobs to professionals and its worth whatever they charge...