Rocks ain't free. Finding and picking up a rock usually requires at least half a minute, so if one takes an hourly wage to be $24 that makes the cost of obtaining a rock equal to... about 20 cents.

But there are two further considerations:

(1) The cost of the rock drops precipitously with your ability to pay. No matter what value you compute for the rock, you will find that the actual cost of a rock never rises above "negligible" for anyone who isn't severely handicapped, regardless of ability to pay.

(2) If you anticipate needing a rock in the future, you probably don't have to dedicate any time to finding one. You can just pick one up whenever it happens to be convenient.