Tires...
A less expensive option that works is good. Tires can be one of the bigger expenses. I've said it before but the guy who ran Paul Tracy's karting team (then went to a truck series team) said the "3 Ts" were the most expensive. Time, tires, travel. You can't use street tires like in hornet or hobby stock. I wouldn't anyway unless the speed ratings were suitable. At that point you're into the cost of a solid entry or mid level race tire. The short track and most of the karting orgs use a limited supply model. The downside for the dealers is that they get squeezed out in favor of a single retail vendor. At the Bullring you can buy a set of tires at the top of the season (or your first race if you run part time) then for every race you run you can buy two tires. Depending on the org the tires are serialized and/or stamped/branded and checked at tech. On a practice only day tires are open. On a day with a points race or heats it's your allotted tires only. That's an example, I don't think the number of tires would work for road race but for an entry level, regional only class some bowling ball tires in a limited supply may appeal to those starting. If nothing else driving on hard tires teaches you how to drive. Or how to fix your damaged car when you crash.
Costs...
Compared to other entry level forms of motorsport SM is expensive. There are some fixed costs, like track rental that make it more than for example circle track. You can get into hornet stock or mini stock for a couple grand for the car plus safety (one track around here has a $1k claimer). Four point HREW cage, entry level safety (no head and neck), street tires, pump gas, no mods. Mostly for a fun run. Some tracks if you win more than a couple/few times no more points. If you are too strong they boot you up to the next class. Hobby stock is the next rung up, about $3k-5k per car, limited mods, street tires, six point cage, all safety gear, pump gas, real tech. We could hit 80 mph on a 3/8 mile banked bullring. Payout each race and end of year. Short track entry costs are $40-50 bucks per pit pass. For $25k you could run regional late model and make enough in purse to pay for most of your entry, tires and fuel. Barrier to entry is low, get a "license" or track membership and you're in.
Off road is a bit more. Short track mini stock is about $4-5k to build a competitive truck. Limited shocks, limited travel, stock unmodded engine, 6 point DOM cage, full safety, small (for offroad) 31" DOT tires. Entry about the same as circle track plus a ton of contingency, tires, shocks, oil, wheels, fiberglass body panels, etc. The trucks are tame enough not to break much if built right but still grab a fair amount of air and not too bad in the speed dept. You can get a championship, sorted, winning truck for under $10k. Desert is more, along the lines of SM in both vehicle cost and entry for entry level racing. Limited buggies are about the cost of short course truck. What's common here are people share ownership of the car and program. The big boy classes, fast buggies and bigger trucks are well into six figures just for the truck. Barrier to entry is low. Get a car and come racing though some guys have more money than talent and enter a big class for the first time in something like the Baja 1000 and no good comes of it. In that regard I have a good appreciation of the licensing model of club racing (and shifter karts as well). Easy to join the low HP, entry level classes. Prove your chops and decision making skills before you move up.
Desert off road doesn't have that but short course does.
An issue with using a Miata as a donor, even a late model car, is cost and availability. A grassroots, ground floor entry needs to be something you can get on the cheap, not real powerful but still build into a safe entry level car. Something like a late model Mazda or Honda sedan, front wheel drive, no frills. You can find those salvaged or with blown motors for under a grand, many closer to $500.
DE...
DE needs to be its own thing, not necessarily on a race weekend. There is no club road racing here but there are two supercar tracks, busy most of the time. Spring Mountain, the track country club has sold out DE for both members and non members. (or they did last time I looked...) The tuner car thing here is still big. There is a problem with kids racing on the freeway on the weekends. When I looked at solo here it was doing pretty well. It was the only club event in the area. As a comparison, there are 4 short tracks in a couple hours drive. Except for July/August when it's hotter than the face of the sun you can run all year round. SCCA does have a DE/TT program but compared to NASA it's not well leveraged and has spotty participation. With the still popular Fast and Furious, the tuner car kids and the like there should be a strong market for affordable track days.