Generally retail has a markup of 8x over COGS (parts cost). For DIY we pay retail for the parts, not wholesale, so our COGS is usually 2x what a company can make. So a quick and dirty value calculation for DIY is 4x COGS for us.
If it costs us $500 to build an amp, it's generally comparable to a $2k retail amp.
I think this can really vary a lot. As an extreme example, consider replicating something like a Denon AVR-S670H 5.2 channel AV receiver for $550. You'd have a hard time building just the enclosure for this price.
If you wanted to build a basic stereo Purifi 1ET400A amp with a single Hypex SMPS1200a400 power supply, cheap chassis, and simple op-amp input buffers, it would cost you about $1000 or so. VTV sells this same configuration factory-built with a warranty for $1100. But this kind of product involves connected preassembled boards, so you are already paying a lot of the markup from the parts cost.
The First Watt amps are probably the best example of Tyson's 4x formula. These generally sell for around $4000 new from the factory. You can probably build one with a simple chassis (the factory chassis is not that fancy anyway) for about $800 to $1000. But that is assuming you don't make any mistakes in the build and have to replace parts, and spend time shopping around for the best cost on parts. Electronic components have gone up in price a LOT in the past few years, so this could be an under estimate.
But the First Watt amps aren't really designed as commercial products in the same way as the Pass Labs products. They are Nelson Pass's contribution to the DIY community and a way for him to experiment with different amplifier topologies.
Higher end products are where you will see the most mark-up over parts cost. I'm talking about amps and preamps that sell for $10K or more (sometimes much more). These are (were) not produced in very high volume, so the manufacturer has to cover their overhead (including engineering, marketing, management, facilities, etc.) with far fewer unit sales. It takes some engineering know-how to be able to replicate these kind of products, but the schematics of many older (10+ years) products can often be found. And some of these would still be considered outstanding products today (particularly the tube based products). The challenge with many of these designs is that the original parts used are no longer available, so an ability to figure out appropriate substitute parts is required.
If you want to get your feet wet with DIY, I'd suggest one of the Elekits. These sell for between $500 to $2200 (plus the cost of the tubes in the case of their higher end 300B amps). They are well designed with excellent build instructions, and are comparable to commercial products that are 2-3x the cost. Yes, you could save a little money by sourcing all the parts yourself, but that can take a lot of time and effort.
The ANK kits are also very nice products, but are pricier, not as well documented, and often have issues with missing parts (although the company will generally rectify these shortages as quickly as possible when made aware).
