Originally posted by ll_coffee_lP $10k is ridiculous even if you are making money with it...
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As a business decision, I always think it's a good rule of thumb that if you can make more than your investment in equipment than it's not a bad investment, especially if you can recoup your expenses before the camera loses too much of its initial value. This is why Aps-C is the best cost to performance ratio for most people who only get photographic work infrequently. If you get regular photographic work every week, then full frame is more reasonable. If you regularly shoot for nature magazines or art exhibits and have great insurance, then medium format may be for you.
On the other hand, if you're a well heeled hobbyist, do whatever the hell you feel like.