We'll be going to Mexico in a few weeks on a short holiday. We'll take some US dollars in cash (I happen to have some left over from a previous trip, and did not convert them back to GBP).
For the main spend we'll take credit cards that don't charge a fee for each foreign currency transaction, and be sure to rack up all expenses in local currency rather than pay the exorbitant rates to get them billed in GBP.
I'm not sure any of us can answer your question, as none of us knows your budgets, what you like to eat, or drink, whether you need to include accommodation and transfers between airports and other locations in the budget.
We don't know where you will be staying - backwoods places off the tourist trail will be much cheaper than Cancun or Mexico City, for example.
I see from a previous question that it's likely to be Mexico City.
https://www.theanswerbank.co.uk/Travel/Question1588223.html
Some good answers were given there, and some of those links also include typical prices.
Having said that, not eating in 'fancy restaurants' could mean almost anything. To some people, a fancy restaurant has three Michelin stars and would cost upwards of GBP150/head (anywhere in the world). Or it could mean everyday restaurants used by locals at below GBP10/head.
The best answer is given above. Work out a daily budget to include accommodation, food, drinks, excursions, car hire, gifts for people back home (or yourselves), taxi transfers and so on. Multiply up to get a total, and there is your estimate.
And, just curious, but why go to Mexico if you don't like Mexican food? I have to say that as in almost every place I've travelled to, real local food is very different from the international version of that local cuisine sold in various 'Mexican' restaurants.
In any case, have fun - and enjoy yourselves :)