Dear new Start-Up Company,
Thank you for the offer but I cannot justify shooting your exciting new product line on a trade basis. I'm a small independent artist who does this out of love, not money. I fear that if I give you my time & efforts, you will not respect me in the morning.
I'm flattered you like my work but the promise of exposure is like over-usage of the exclamation point. Everyone uses that promise & I've NEVER met anyone who can actually deliver on those promises. If you can prove to me that you have the right connections &/or back it up w/ facts & statistics, I'm open to reconsidering at a later date.
Also if you're going to turn a profit, specifically based on the images I produce, I think its only fair I get paid. No, I am not inclined to work on spec either. Yes, I'm sure you're an honorable fella but I've been in to many instances where Person X promises L, M, N, O, & P only when it comes to crunch time, I'm the one left without a chair when the music stops. I have to protect myself. I'm sure you can understand.
Another reason why I'm declining your generous offer is that we do not have a preexisting work relationship. My (admittedly lame) justification goes like, "Well you already helped me out in the past when I needed it, I can justify helping you out now." Its my way of giving back.
I'm willing to negotiate a price that's not only fair & respects my time & the quality product I deliver but also is within your budget. My point being you've got to give me something to work with. I cannot draw blood from a stone, nor will I bother trying.
Its nothing personal.