How much can I sell a custom 2000 Honda Civic Si for?

You most likely wont be able to get all of your money back. Add up the costs of all the mods and take 20% of that and add it too the kbb value. Thats what I would see as a realistic asking price. You could probably get more selling all of the mods individually but it will take a tremendous amount of time and a lot of work to replace all the engine mods with stock parts. It wouldnt be worth it in my opinion, but if youve got a lot of time to spare it might be.