Opening crates is the best way to get the parts and blueprints. If the crates give you back dollars for extra blueprints though, that money can't be used directly in the chop shop. You have to buy a part e.g Black Market and the conversion rate is low. I also seem to only get roughly the same 8 parts showing in the Black Market, no hyper parts. I have a couple of legendary items and loads of epic items in the chop shop but when I hit the salvage button at the bottom to scrap them and convert to currency, none are worth more than about 1300 tokens each. I have over 40 upgrade parts (some are x5 units each) in there from hundreds of races but the total value didn't come to 40k tokens, which isn't enough for a single Porche 911/991 BP.
The upgrade parts I win from the Tuner Trials but I get 1-2 parts per race. This conversion rate means I have to do nearly 100 Tuner Trial races to get a single blueprint or 2500 races to unlock one car and I can only do about 2 per day, sometimes 10 if it's all access.
I'm going to stick to opening the crates as that seems to be the best value way to get upgrades although it costs real money for the gold ones. I will also do more Blackridge races as that can give 5 gold crates. But again, there's too much grinding with those. It's 12*5=60 races for the first crates and then another 60 for the gold crates.
It's not even as if this setup encourages people to pay more money because gold can't be used in the chop shop. It just makes the game tedious to play through. If I could unlock more cars then I could race more and that would encourage spending more gold because I'd have more races and parts to unlock.