SIM card physical, mechanical, and electrical contact specifications require ~0.05microns (~50nm) of gold plating on the electrical contact pads. (As defined in ETSI TS 102 221 V11.0.0, although 3GPP/3GPP2 USIM interfaces adhere to the same standards for convenience, intercompatibility, and raw mass-produced cheapness.)
0.05micron gold plating (over copper) is basically the minimum possible in mass-produced PCBs (thinner gold platings are possible but actually cost more to apply consistently). And it's basically rated for ~25 insert/remove cycles (under the rated 0.5N contact force) before the plating strips off. The exposed electrical (copper) contact will continue to function afterwards, but when mated with a dissimilar metal it is highly susceptible to a type of electrically-accelerated oxidation known as galvanic fretting ... which greatly reduces the functional life of the SIM card. (It's the same electrical interface used on credit cards, bank cards, and other such stuff, although they require greater thickness of gold plating because they're intended to function for well over ~500 insert/remove cycles.)
Similar standards "recommend" a minimum ~0.2micron (200nm) gold plating on the SIM card receptacle (built into the phone), basically rated for ~100 insert/remove cycles before too much plating strips off. Although phone manufacturers have more leeway, they don't even have to use gold if they can find cheaper alternatives which will meet the required number of insert/remove cycles across the product's expected (~2 year) service life.
You can safely bet that manufacturers will never use more gold than strictly required. Not when they want to make profit on SIM cards they sell in bulk at less than $0.10 per unit.
