The biggest bargains are Booze. Smokes, Vegetables, and almost everthing is at least half price.
You should hear from a gentleman that goes by the handle "Kino" and from what I have gathered is in Monterrey and has been for many years.
I have only ever passed through 10-15 times
I presume things can change in different parts of the country, but gas is in the $2.20 per gallon range in Baja these days.
Beer is usually around the equivalent of $1.50 to $2.50, but sometimes you can find it a little cheaper during a happy hour, etc.
If you order "national drinks" they will be cheaper; depending on the brand, as low as $1.50 to $2.50 a mixed drink. That means with booze from Mexico. You will find that foreign brands are not cheap, (say $4.00 to $5.00 depending on the brand) and sometimes particular brands or taxes can drive prices even higher.
Taxis can be tricky because there are several different kinds of public transportation. Regular transit busses will usually cost you around 50 cents. Routa taxis (on a regular route) can cost $1.00 to $1.50; and Taxi Libre (taxis not a route, not free) will cost $3.00 or more, depending on the zone or how far you want to go. There will usually be a sign in the front window telling where they are going.
Taxis from a bus station or airport must have a special license, and they charge more. Always ask the driver for a set price before you get in.
Depending on the type of club or restaurant, the prices can go up from there. The nicer the venue, the higher the costs... and there are some very nice restaurants and clubs where you will pay plenty. On the other hand, you can also find a taco for $1.00 to $2.00; ceviche tostadas about the same in neighborhood stands.
You will find many restaurants advertising the "comida del dia" which is their set menu lunch for $5.00 to $7.00 and it will include everything. Nicer restaurants... higher prices.
Obviously there are "specials" and "promotions" where you can get other deals. Tourism is down, and if you are at a location that depends on tourists, they may have some better prices... but as you can see, most are pretty reasonable.
The current exchange rate as of today is about 13.4 pesos to the dollar, related to the prices above. So if something is 36 pesos, that is about $3.00 US. Keep small bills with you and never pass up the chance to change a larger bill if you are at a gas station or market where you have a chance. Places often don't have change.
Monterrey is a pretty cosmopolitan city compared to many others in Mexico... at least by reputation.