Why do you think you'd need to rust blue it? You might mean hot salts blue, which is what S&W blued revolvers are; rust bluing is a different, generally more expensive process.

Is $400 more than you'd like to spend all in, or just for the process itself? You have to factor cost of shipping into these, which can run upwards of $100 total.

Otherwise, murphydog's right -- standard factory reblue for $220, or heck, throw in an extra $55 for high polish (what I'd do).

For something less expensive, especially if you can find it done locally, parkerizing would work.

For very cheap, you could get a do-it-yourself spray on material like duracoat (hate to suggest that for a wheel gun, but that's just me -- to each their own).

Or just enjoy the history it has accumulated.

But quality finishes tend to cost, and you tend to get what you pay for.