For the quantitative aspect of the situation, you should examine their offer and try to figure out how they came to this figure. What is it in your company that can make them money? What synergies would you provide them? Do they have a solid marketing team to sell your product? What can they do that you can't, and what can you do that they can't? That way you'll have an idea if their offer is reasonable.
Now, let's reverse the question. How do you value your own company? Do you have the resources and know-how to make it into a $5M company? $100M? You've worked hard up to this point, but this is in the past now. Are you confident enough that you'll be able to get to a point where you actually make good profit?
Now the important point is that if they value your company higher than you do, you should sell. Period. The fact that they are offering something for your company is no guarantee that you'll be able to make nearly as much on your own. They might be willing to give you a lot because you're worth a lot to them. You might not be worth so much without outside help.
As you mentionned, money is but one factor in your decision. Still I hope this helps a little...