Depends on your budget and your intended use, really. If you're building a 'casual' or 'hobby' kind of rifle on the cheap, then you'll be fine with TNA level stuff.
When it comes to a BCG or other major AR components, you do get what you pay for. The cheapest BCGs will have batch or no QC inspections, cheapest materials, cheapest coatings and components. If you're only using it a few times a year, that's usually just fine, and it may never break or fail on you for the life of your AR. But buying at this level involves a bit of risk that your stuff might fail or break on you.
If you want something with a level of quality that you can depend on, you'll have to shell out some more money for a quality BCG made by Colt, BCM, Daniel Defense, etc. These BCGs are made closer to the mil-spec TDP and have far more QC, gauging, tighter tolerances, inspections, better materials and coatings. If I wanted a BCG to last, to work reliably especially with frequent use, then I'd buy from this category. "Buy once, cry once" applies here.
Then there are the 'exotic' level of BCGs and other components, where you're paying for a style or a certain look made by a certain company. These BCGs may work just fine but they tend to have a higher price tag because they use some proprietary technologies or designs or coatings to squeeze extra performance from your rifle. Whether these claims or true or not, or whether you consider the extra cost worthwhile is up to you and your wallet. For my money, I consider this level a bit gimmicky and not worth my money. But to each their own.