i had the same question when I was researching what turbo to go with. Ultimately I made a excel sheet comparing everything and including an itemized list of what the kits include. Side by side you will be VERY surprised at how similar some of the kits are. I even found that they have great discrepancies in price for the spacer plate, 50mm tial bov, and air to air intercoolers. I am sending this from my phone, but I can give you the excel sheet later if you are interested.
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That is valid until you get to quality, fit, finish, and customer service. I can remember in the beginning when Velocity kits were total pieces of sh** - the charge tubes didn't line up, the welding was pre-kindergarten level, etc. Of course, that changed, and in the end, they got better. Now, when things are on a more level playing field, you should be looking at the specific parts in the kits: quality of the header - are they known to crack? the turbo - is it a Garrett, or a Chinese knockoff? There are differences in components that must be looked at as well. What exactly are you getting for your money? Would you be VERY surprised at how similar a Honda is to a Rolls Royce? Similar does not mean they are the same . . .