Let me just state that I don't want to be the "negative" post or the one that has a slightly negative tone about pro-bono work, but I do want to offer some real world observations about pro-bono work that we have experienced.
First, we believe very much in giving back to the community and over the years, we have offered both "reduced-rates" & "pro-bono" to some charities & non-profits, but we have also been used and abused by some groups in doing this. This has led us to be more particular about who we actually offer "pro-bono" to and to state EXACTLY what services are being offered.
Very rarely do we offer the pro-bono service now because as it's been stated above, "Free has no value." but instead we offer "reduced-rates." Where it is true that SOME non-profits are limited in budget, it doesn't mean they all are. As we have learned from some of our accounting clients, "non-profit" is a tax term mainly used for IRS purposes. Just because the organization is using it, doesn't mean that they don't have the resources to pay you for your work.
It has been our experience that if the organization is well established in number of years in existence, there is usually a budget in place that has allowed them to stay as such, a budget that can pay for your services.
Don't be afraid of being honest that your services do have value and that you would like to be paid for them. Some organizations will use the non-profit banner knowing that they have the resources to pay you, but will still expect you to give them a break. A sign of being a true business person is to do what you did and walk away from a deal that is not going to benefit you.
Pro-bono doesn't mean that you can't receive benefits from your contributions, so you should make sure that the benefits you want to receive are well stated.
Heck, even our lovely government gives you a deduction on your taxes for being generous with a donation and they're the biggest NON-PROFIT there is!!!