My dime's worth
I think a donation of any kind is wonderful for a chapter, whether it's a donation of time or money. Some alumni (referring to both men's and women's groups here!) don't have a lot of extra money, especially younger ones trying to establish a career and life outside of college. They may have more time to help out and be actively involved. Older alumni may be busier with career and family, but have financial resources to make monetary donations to the chapter.
Remember that a collegiate chapter depends on area alums to support them. Chapters need dedicated advisers and (house) corporation officers to function successfully. Chapters depend on an active organization of area alums for support, especially when it comes to recruitment! Be actively involved where you can, whether it be as a member of your local alumni chapter or as an adviser or officer. If you're a collegian and see a lot of alums on a regular basis, I'd bet you will be more likely to remain involved when you are an alumni.
Most, if not all organizations have a Foundation on the national or international level. There are different tax laws and such for donations, so a donation I make to the AOII Foundation may be tax-deductable. However if I make a cash donation to my collegiate chapter or corporation board I believe it usually isn't tax-deductable. Of course, check with your personal financial adviser if you want to donate money and get a tax-deduction!
One last donation tip, some employers will match your donations, up to $X per year. Check and see if your employer does so and any requirements (such as must be a tax-deductable donation or whatever). If they match, make sure to take advantage of that when you make your annual donations to your organization's Foundation!
Christin