Parts car cost in budget?

spares are kind of a grey area. The by the book way it works is like this. After each race you're supposed to ask for a residual value on the car, and then that is what you can spend to fix/upgrade. Residual value should reflect what is broken. So if you toast an engine, maybe your value is $100 and you have $400 to find a new one. Then you can play the budget game of buying a parts car, selling a lot of it, and showing you got the drive train for $100 all said and done. Document it all and at the next race you can prove to the judges that you're under budget.

However, If you blow a motor at the track, no one comes stopping you from swapping in a spare until you prove your budget, you just plow ahead and try to win that heroic fix. So keeping a spare motor, trans, hubs, suspension, etc in your trailer never gets another thought. With the huge exception that if you swap out for a clearly cheated up high dollar built engine, someone's going to raise an eyebrow. That said, preemptively gathering spares is very common, and encouraged. Grab your parts car, take the parts you need, sell the rest, keep that documentation on how you got them just in case. 

How much you get sweated on this stuff comes down to your car. If you have some super cheaty spec E36 and you're swapping motors left and right, you might get scrutinized. If you're running something french in C-class and you show up with a trailer full of spare engines "just in case", no one will ever ask you to prove you got them all inside budget.

THIS!!!!!!!!

Do not overly sweat the budget thing unless you are cheating it.  If you have a car like a Miata, Porsche, BMW, etc they will probably want to see receipts.  None of my cars have they ever cared to see receipts because they have a stock suspension, stock motor, stock nearly everything.  So if you are raiding the go fast aftermarket parts bin then save receipts for everything and have a great story.  If you have a stock 1986 Grand Marquis don't sweat it.

Don't come in like "Roadkill". Actually try.

The "Correct" way of doing things is think about it like "Selling" yourselves the parts.

So you go buy an auction car, That costs $X. Then you take the parts you want off and sell the rest.

That money you get is $Y, with Y being less than half of what you think you should be getting for the scrap parts.

X-Y= the cost of ALL the parts you kept and put on the race car.

If by magic X-Y=$0, you've already won more than you ever will in Lemons.

Document every transaction. Line item all the parts separately with the Cost for the car and scrap income. Then list THAT as "Recovered Drivetrain" on your main list.

What you "can't" do, if by some crazy magic you have extra Profit and don't just take a huge bath on the whole thing, Is roll that into the rest of the build. The auction car > Parts Harvest > Scrap is its own self contained entity. Bonus points if you set up a Lemonade style stand  with a sign like "Bernie Madoff's own slightly used cheap race car parts" or something and video or photograph one of you buying the parts from the other. Then include that in your budget evidence.