A good question to think about.
If your old heads are like mine (124K miles on them), your guides may need bronze liners installed. What is the typical charge, $15 to $20 per guide?
The exhaust valve guides need to be cut down then machined for valve stem seals. If this costs more than $75 I would buy the two cutters and do it myself.
The exhaust valves need to be replaced with ones that do not have that groove. About $110.
Of course a valve job is a good idea. Assume $225 to $275 depending on the type of equipment and local market?