Welcome to our forum.
If you require neck/shoulder jets you have a couple options...like you mentioned look at something reverse molded, keep in my it is 100% imperative you sit in the actual model to make sure you fit comfortably in that particular seat and make sure you aren't too tall for it.Another option would be to look at the Bullfrog line if you have a dealer nearby, using the NeckBlaster or NeckMasseuse JetPak which you can move around into various different seats which are at different heights so that you and any family members of varying heights could use it.
Considering there is around 250 feet of "plumbing" inside a jetpac @ 1.00 a foot plus the jets and what not, hardware/labor to build the jet pac, should equate out to 350.00 or so per. We sell extra pac's for 295.00 Quick story. We recently had a jetpac fail, one of the jets quit working for what ever reason. The customer brought it in and good lord there were hoses, fittings elbows and jet psc flying all over the place. Looked like an octopus fighting with a snake. Luckily no one was killed or lost an eye. I thought quickly and grabbed a temporary one off the shelf, and sent him home with it, till we can get his (under warranty for another 3 years) shipped from the factory. No need for a service call. no need to have a tech go out and maybe even have to drain the spa to replace the jet back. The customer was ecstatic beyond belief as his last spa an Arctic, and it did have to be torn into by a person like yourself to fix the problem. So yes about 300 to 350 a pac. Curious what you charge Tman per hour to dig into a spa and replace a jet or manifold, perhaps a divertor?