Having had some experience selling modded vehicles, both basic bolt on stuff and highly modded, general rule of thumb is about 25% return on the dollar for mods, assuming you are selling private.
Selling to a dealership / trade in, zero increase and generally get better or same deal if put it back to factory.
Selling privary party with ton of mods, can get 25% but it will take longer to usually sell as very specific type of buyer who is looking for a vehicle that's already modded. They are def out there, sold 3 well modded vehicle in past few years but can take twice as long to sell as well vs light mods / stock, assuming your price point is reasonable.
Quickest way if doing it private party is usually put it back to stock and then sell the mods yourself.