Have you seen how much GW are charging for each SS/PW warrior? If they released a model it will be in a metal blister on account of how rare arbites armies will be so that's a good indicatition of the likely price.
If you are building an arbites army, why don't you just convert them from scouts? You got shotguns and carapace as standard, even some the heads would work without any need for conversion. You can get combat shields off eBay for a bargain if you are lucky, which will do for riot shields. With a file and only a little work, chainswords can be made into decent looking power muals.
The only tricky part would be the heads. If FW sell Elysian heads seperately, you are good to go, if not, you might have to bite the bullet and buy a few squads worth. But if you resell the bits you don't want or need on eBay you should be able to recoup much of the original cost and might even make a little profit if you are lucky.
With a little imagination and effort you can build a very convincing army for a reasonable price and maybe even have some fun with the converting.