About what would the cost be to replace the tappets?

The cost to replace tappets can vary widely depending on the make and model of the vehicle, labor rates, and whether you're using original equipment manufacturer (OEM) or aftermarket parts. On average, you can expect to pay between $200 to $1,000 for the parts and labor combined. It's advisable to get a detailed estimate from a trusted mechanic for a more accurate figure based on your specific situation.