Not really
1 lb ground beef is about 3 bucks on sale
cheese is a couple bucks
sauce is a couple bucks
Shells about a buck or two
Figure this can make 8 tacos or 6 burritos, not counting the tomatos or beans they also use.
Figure it will take about a half hour to brown the beef and make the tacos
3 + 2 + 2 + 1 + 1/2 hour of labor $20.00 = 28 bucks for 8 tacos
Problems with this "math"
A) As mentioned you shouldn't be factoring in $20 to make the tacos, especially when you aren't factoring in the time to drive to a taco bell, order, pay, drive home, unwrap.
B) If you use soft corn tortillas for tacos you can make hundreds of them for under $5. The cost of them is negligible.
C) Cheese is only a couple of bucks if you buy shredded cheese and even then only if you buy it at a corner store or something. Costco has enormous quantities of shredded cheese that costs way less than $2 per 8 tacos.
D) What sauce are you using exactly and what is the supposed equivalent at TB? Are you talking about their sauce packets? You can likely get a vat of that on the cheap.
My guess is that the real world cost would be close to $4-5 at worst (assuming you don't buy meat from places like Costco) with the "labor" being a wash compared to driving.