Selling text link spots and putting a value to that particular link spot are indeed subjective. The value of your text link ad spot depends on placement, traffic, type, size, etc. Some advertisers place high value on Google PR and DMOZ listing; you may want to include these in your valuation.
To make my life easier, as a rule of thumb, I use TLA’s calculator located at:
Also, you can cross check against LinkWorth’s:
Do use these calculators as a guideline only and not to value your links solely based on what these calculators churn out. Always remember that advertisers will expect substantial discount for a long term advertising duration as well as multiple site advertisements. For returning customers, don’t forget to give them a discount too; they may actually list additional sites (happens to me).