There are a couple of ways to go about it. Custom parts from start to finish but that gets stupid expensive. OR. Find a used charger and modify to your liking. PMACA chassis and AR tube and your done. OR buy the receiver then part it together over time.About what does it cost to build a charger like those? I wouldn't mind building one some time. I wouldn't even know where to begin though
If you use factory Ruger parts you can really keep the cost down. The PMAC chassis are a little pricey but the occasionally come up on ODT used as do buffer tubes. Having said all this $350 is probably the cheapest you could go all the way up to a ridiculous amount of which I will never get the money back out again. The accumulation of parts is part of the fun so my recommendation would be start with the receiver that you buy from s&p and shop for parts.