You might need to change your idea of what a sale is.
I can't be sure from your description, but I think that I have been in this position with many of my startup clients. When you say that they want what you are making, what you really mean is that they would want what you are making if you actually had it--which you don't, at least now when they want it. So, you need to differentiate between whether you just need one or two early clients to get you over the hump or whether this is an endemic problem to your business that won't go away.
So, ask yourself this, "If you had already delivered this to one (or two or three) clients, would you still have this problem?" Is this investment and delivery cost a startup problem where you need to get over the hump or is this something that faces you for each and every client forever?
If it is the latter, you obviously need a strategic investor and you will need to give away a significant part of your company pre-revenue to get that. But if this is a one-time problem, you might need to change the idea of what a sale is.
In that case, a sale is not to get a client to accept a proposal for what you wish you had. A sale is to get a client to buy what you actually have, which is an expensive, slow solution to this problem. I know that once you get over the hump that you will be able to sell it cheaper and faster, and that your experience with the proposals so far might give you a waiting audience, which bodes well for your business.
But your first (or first few) sales have to be focused on the problem at hand, not the going business you hope to later have. It's all about cash flow. So, you need to start looking for different clients than you have written proposals for and you need to write a proposal for the $500K or (gasp) $1.6M that you really need. That means that you are looking for:
- A big company
- A company who is dying from the problem you solve
- A company for whom solving this problem is worth multi-millions
- A company for whom solving this problem is a competitive advantage
And you need to treat them more as a partner than as a client. It could be that you need to actually make them a partner in your company, but maybe not. You might be able to get away with giving them exclusive use of your product for three years within their industry or giving them a list of other companies you agree not to sell to withing that time frame or something else that gives them the advantage that they need.
In other words, you need to find a company for whom the pain is so bad that they will put up the money you need and wait the time you need. It doesn't matter that you have found companies that will buy when you are a going business. What matters is that you need to find a company who will buy what you can actually sell now. Perhaps you could go back to a few of the 10 companies you approached in the past to see if they would be willing to split the startup costs and wait if they each get some kind of exclusive in their industry. Or maybe you could find a company who is a channel for the companies that you eventually want as clients--if you are selling a PR solution, perhaps you need to find a big PR agency that is willing to sell you into several of their clients in return for an exclusive for a few years.
But I think you need to change the way you are thinking to match the proverbial vegetable guy. Each day he sells what he has on the truck today. He doesn't tell people he will have avocados next week. He sells the cucumbers he has on the truck today.
Sell what's on the truck. That is when you find out how good a salesperson you are. It's easy to feel like a good salesperson when you just offer people what they want. It's harder to sell what you actually have. When you do, I think your problem is solved.