Professionally built "spec legal" and COMPETITIVE engines are more on the order of $7k. If you just want to run, you can run with an $800 salvage engine. If you want to run with the big dogs... that probably won't cut it. The difference can be on the order of 20% more HP to the wheels!

A heavily raced Miata will probably consume over the course of a season or two: clutch, clutch slave cylinder (a weak point on the Miata), brake pads/rotors/calipers, axles (CV joints and/or bearings), front wheel bearings, spark plug wires, and tires. ANYTHING else can give up, though... it's a 20-year-old car and you're racing it! Clutch/brake master cylinders, clutch/brake hoses, water hoses, belts, transmissions, differentials, radiators, alternators, water pumps, ball joints, tie rod ends, bushings, etc.

Regarding Lemons/Chump costs, plan on spending $1000 for a weekend of racing and you usually won't be disappointed. That's entry fees, transportation, food, lodging, car expenses and everything. $700-1000 usually does it, and you'll get about 3-4 hours of quality racing time out of it.