Usually you would have your own nursery through which you sold the plants and got money back for your travel expenses and made a living out of the whole thing that way. One who is currently pursuing this with a focus on the region you have mentioned is Ian Barclay at desertnorthwest.com.

Or you would have some kind of arrangement with a large wholesale nursery looking for new introductions, with the resources to compensate you for your contributions to their product line.

Or you would have a substantial income from elsewhere and pursue plant exploration as an avocation or sideline.

I have also tried looking for more information in regards to seed collecting in Mexico but there is not much information. There are several nurseries that introduce new plant material and have gone on expeditions to other countries(an example would be YuccaDo Nursery). If you grow caudiciforms you might have already heard of Guy Wrinkle.

It also might help to have a specialty in a certain plant genus or category of plants, so you know what to look for. The cost should be reasonable if you are traveling by car from Southern CA or some of the border states to Mexico.

This is something I would like to do as well one day and hopefully it is not an impossible one. If you find any more information, feel free to post.

Here is a link that might be useful: Plant Expeditions