We use money as a medium because barter simply doesn't always work. Imagine you try to barter your photography services to a photographer...not very valuable, eh?

But, when you think of your services, I'd encourage you to think of them in terms of dollars. What is an hour of your time as a photographer worth? Let's put a number on it - say $100/hour.

If you would have spent 4 hours doing this shoot, then that would be equal to $400 worth of services.

Is Yoga Guru offering something that valuable in return? And when I say "that valuable" I don't simply mean the retail value. In a barter system - the value of an object or service is its value to you, not to anyone else.

Even though Guru may have offered you $400 in workshop attendance, if you cannot actually attend them, they're value is nil to you and this becomes a one-sided trade. I wouldn't feel guilty at all turning this down.

It is very easy for new artists to be suckered into barter deals that are very one sided - especially the trade for "exposure" as mattdm points out. Be very, very careful about trades. Make sure that you always look at your services in terms of dollars and convert their offer back into dollars worth it to you. Make sure you only trade evenly, less you have a ton of "exposure" and workshops and an empty bank account at the end of the day.

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Additional point on guilt: You don't owe anything to anyone, except the ones you've promised work. Someone can ask you to do a shoot and you can explore the idea/subject/work. But, if you're not interested in the gig at the end of the day, you don't have to take it. Photography is your skill - it has value - and you get to pick where you apply that skill. Don't feel guilty about this, ever.