Disclaimer: this is how it's done in the US, I have no idea about Mexico but this may give you some ideas how to get there. Save copies of any documentation, if this is ever questioned at least you did not use the PFA method, and for someone to say you were wrong, they'd have to find better documentation which probably is not worth the effort down the road.
To figure the ratio of land to improvements, find a couple vacant lots that recently sold, that are similar to yours (ocean front if yours is ocean front, etc, and close in proximity). Lacking recent land sales you could use listings, but I'd deduct 10-20% unless people typically get their asking price in your area. Then do the same with houses, get your best evidence as to what your house would sell for. Subtract the estimated land value from the total price and that's the contribution of your improvements and there is your percentage.
In the US, land in a typical subdivision is going to be somewhat less than 1/3 of the total price (custom lots, view lots, acreage can be different). Could be as low as 10%, depending on the area. Built up areas with no vacant sales, homes that have been around 50-60 years, much harder to estimate but can be done.
On a condo, there is no land value, you own "air rights."
I don't know if appraisals in Mexico are anything like in the US, but in the US most appraisals will estimate the land value in the cost approach section. So you could figure the percentage it was at time of purchase, that is likely good enough for tax purposes, unless you've owned the place for 40 years or something.
You might also consult one of the local realtors who might help come up with some numbers, maybe they can refer you to a local appraiser and you could see what they'd charge you for something in writing. Good luck.
