The tax value appraisal is called (Impuesto al Valor Catastral) and will not require an on-site property inspection. This value is set by a county appraiser (Perito Valuador) and it is usually a fraction of the commercial value or the selling price. This appraised value is used by the Notario to determine the closing costs and the annual tax payment (Predial).
If you are going to be getting a bank loan on your Mexico property you will need to have two appraisals done. One appraisal will be for the tax value of the property and the other one will be for the lender to determine the commercial value.
Normally the process for this second appraisal is done with an on-site inspection and the "comparable" value of properties sold within the last six months.
In the United States a "Multiple Listing Service" is used to pull these comps. Mexico currently does not use this system in most of their cities so it is not as automatic as in the United States or Canada.
The banks only work with trusted appraisers of their choosing because of the chance for a wide interpretation of value of the property. The cost of an appraisal can start at $800 and go up depending on the bank, the appraiser, the size of the property and the difficulty of the appraisal.
Your real estate agent can usually tell you what other properties have sold for in the area so this way you can determine the value of the lot and then determine the value of the house depending on the level of the construction, quality of the finishing and size of the house.