A Mexican cruise can be a lot of fun. The weather is nice and you can just stay on the boat and use the pools if you want to. The ports are just OK, unless you plan to spend money to do adventurous tours.
Honestly, I wouldn't cruise on Carnival. They are cheap for a reason - much lower quality than a lot of other cruise lines. There are still other lines cruising out of LA and San Diego for 7 night trips to Mexico. Carnival is the only one still offering 3 night trips. I would go on a better line (Royal Caribbean, Norwegian, Princess, etc).
If you frequently shop at Costco, I would recommend booking through their travel department. Every cruise you book gets you a Costco cash card in return, with the amount depending on the cost of your cruise. It is often between $250-750. So it's a nice little kickback. Pricing usually doesn't vary much between sites for cruises, so the best deals are in what the travel agency gives you in return.
Finally, consider Alaska instead of Mexico if you're planning to cruise in the summer. You might be able to find something within the same budget.