Not to be snarky, but source code, like anything else, is worth exactly what one person is willing to pay for it.
Not a terribly helpful answer I know.
Put yourself in the position of the buyer. I wouldn't care how long it took to write. I only care about revenue that it can generate - be it through licensing, potential ad revenue, or a service it provides to customers, etc.
Ease of duplication is a factor. If it is a blog engine that anyone with a bit of time can put together, it's probably not worth that much. If it's a video processing engine that does useful, difficult, stuff, it's worth alot more.
Pricing is hard.
You have to demonstrate a value to the buyer.
Okay, given your 200K / year number, I'll assume that it would take 2 developers 1 year to replicate it (random numbers).
So, let's say the cost of duplication is 200K (assuming the cost of hiring a dev is appx 100K a year) and the lost revenue of that year is 200K. I would start the price at 400K, and go up from there. If you're going to use cost of duplication as a part of your pricing model, you need to make sure that that's the ONLY way to get the product the client wants. That is to say, there isn't an off-the-shelf solution that approximates it.
Edit
It looks like you're selling the business. In that case, the source code could be an asset, and you need to talk to an accountant. There are a million different ways to value "things" in a company, and depending where you are in the world, there are tax implications for what you decide. You really need to talk to an accountant who has a bit of experience with software companies.