I bought a 4-pack of 100mg Viagra pills for 549 pesos at the Tijuana Costco on Saturday, December 17th.
As I recall, the Costco was posting an exchange rate of 13.2, so if I had used dollars instead, the 100mg pills would have cost $10.40 each at Costco. My bank had a better exchange rate when I bought my pesos for that weekend (approx. 13.8) so I wound up paying around $9.95 per pill.
Crossing times vary greatly depending on when you plan to cross the border.