The cost of the custom arches will vary depending on what you get done. What you will need to budget for is a respray of the doors and rear quarter panel as well. I would say budget about 100 - 150 hours of bodyshop work to get it done, but of course, what you can benefit from is you can get a mould made of the new arches and you can make money off that.
Read this thread through today amd while everything is not too my taste I take my hat off to the quality of the work.
Must have cost a packet if the arches alone took 150 hours at quality body shop rates of £50 + per hour thats a lot of wonga