The cost to repair a slow leak in a tire will depend on what is causing the tire to have the slow leak.
Most times though a slow leak is due to a bad seal between the tire and rim and may just need to be cleaned and re seated and then you might need a valve stem replacement for the tire.
If the tire needs a patch it could cost around $10.00 to patch the tire.
The cost to have the slow leak in the tire repaired though should cost no more than $20.00