I wouldn't do it for less than $50 an hour, personally. I think you're selling yourself WAY short if you do it for $25, especially with you having professional teaching experience in that subject field.
Alot of deciding what to charge would depend on how good of a relationship you have with the "they" that are paying for it, as well. If there is a strong chance/definate future quid pro quo being factored in, I'd take less. If they're rat ---daisies--- who squeeze you for every dime they can when they're selling services to you, I'd charge more. But for a straight-up, neutral buisness relationship, it would take $50 to get me through the door to tutor in a computer subject, one hour minimum, half-hour rounding after that.