Some basic research into the various Spec Miata classes...

RENTALS:

http://www.motorsports-exchange.com/rentalinfo.htm
SCCA or Midwest Council Regional Race (SM or ITA):
(a) $750 Arrive and Drive; second day is $500 - save $250; "Tow it yourself" and save another 12%.
(b) run both SM and ITA (two races, same day, schedule permitting) for an additional $300/day and save !!!
(c) Want to share a car with a friend? Second driver surcharge 250/day;

A small refundable deposit (25%) is required to keep your date reserved. No penalties if the cancellation is received up to three (3) weeks prior to the date of the event.

BUILDERS:
http://www.advanced-autosports.com/
http://www.phillipsraceprep.com/sm.html
http://www.okmiata.com/r_solutions.html
http://www.sandimasauto.com/ [CARS FOR SALE]

USED:
$10k - 1990 1.6 @ http://www.motorsportexchange.com/Dave%20Wheeler's%20SM%20for%20sale.htm

PARTS:
Pro Race Wheels - $99 each @ motorsports-exchange.com

NOTES:

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'90 is good. '91 also. You want a 1.6l, unless you weigh more than ~220# and want to make min. weight easier--then get a '94-'97 1.8l. Don't know exactly why the 1.6 is considered better, it just is. A car with hardtop is a BIG plus. Hardtops are scarce and fetch $900-$1200 on the open market.

Your dream donor car is a high-miles '90 with a messed-up interior (and hardtop) that blew its engine in '99 and now has a well-broken-in crate motor with 20-30K miles on it. Your dream price is $2000. You'll be replacing most of the suspension, bushings, and brakes anyway. Failing the dream donor, high-mileage older cars should be about $2500-$3500. Remember a hardtop is worth a grand.
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The viscous LSD diff on 90-93 cars should likely be replaced with Mazdaspeed Comp version.
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Some 90 and 91 cars have crankshaft problems, but this isn't really an issue since you'll liekly replace it immediately with a crate motor: http://www.miata.net/garage/crankshaft.html
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