Regardless of whether you take advantage of a business opportunity, it's always a good practice to say, "thank you." You never know when you might need to revisit a business opportunity you previously turned down, or ask for additional help with an opportunity you accepted. Writing a thank-you letter every time someone extends an opportunity -- whether it's for a job or the chance to get in on the ground floor of a thriving business -- is considered a standard professional courtesy.
Tone and Format
If you're sending a letter to a colleague you've known for some time, you can be a little less formal than if you are writing to someone with whom you're only professionally acquainted. For example, you can address a long-time colleague by his first name, while you will want to use a formal title for a professional acquaintance. In either case, use personal stationery or letterhead so that you make a good impression. And always use standard business letter format. The letter you write may become part of a permanent record and it should reflect positively on your business acumen.
Tell your acquaintance how much you appreciate being considered for the opportunity. Express your gratitude, no matter your decision. If you turned it down, briefly explain why, such as the timing, funding or your capacity to take on another project. If you simply weren't interested, be gracious and explain that, upon careful consideration, you decided the opportunity wasn't something you could accept at this time. If you think you might want to revisit the opportunity in the future, let the recipient know that you look forward to meeting her sometime in the future to discuss the matter further.