Here's some advice: The word "fair" stopped making sense in kindergarten. You don't get what's fair or what you deserve, you get what you negotiate.
Now, having said that, you have to determine your goals and how to synthesize an ideal set of negotiating levels.
Typically these levels are:
1. Possible Best Case
2. Likely negotiated agreement
3. Worse you'll take (walk away below)
4. BATNA - Your "Best Alternative to Negotiated Agreement."
There are several methods used for calculating acqui-hire or acquisition:
1. Comparables on metrics such as revenue, profit, market share, impressions, downloads, average daily/monthly users
2. The company's (buyer)'s budget - "Well all we have is this"
3. The sellers demands (I won't sell for less than this)
If I were looking at buying you, I'd consider it fair if I Can "buy" you for less than you'll make me, including opportunity costs.
You're going to have give out more (anonymous) details before most people can help you too much:
1. Are you a 1-man shop? Including contractors?
2. How many people at acquirer?
3. How long have to both been in business?
4. How much revenue do you both have? How much profit?
5. What guarantees do you want if you're hired? What agreements are you willing to make (i.e. non-compete)
If you're a one man shop with an app that's generated $30k this year, I'd buy you for $60k and pay you $140k with a non-compete for 2 years. I don't know if you thin that's fair, but it would be to me.
Remember, a negotiation is a game in which both people agree things are fair, or both people agree that they get screwed equally.
After the negotiation, if you have to work with the person, then try to focus on the former case.