Rule of thumb: $10 for every 1,000 followers. Obviously, this varies depending upon a lot of things, but that's a good starting point.
Example:
10,000 followers = $100
100,000 followers = $1,000
This is assuming an engagement rate of around 5%-15%. Obviously, If an account has 10,000 followers and averages about 100 likes per photo, that's going to hurt the valuation.
This is a good starting point. Now, many of us add, or in this case, deduct from the above figures if they are not monetized. thus a 10K account that is not making money would be about $50-75, a 100K account (2k-2.5k likes per photo) about $300-$500.
Many accounts can get 1K likes per photo depending on how many engagement groups they join, so, likes and total REAL followers is not that great anymore.
What makes an account valuable is how much money it's making. And you must have hard proof, no screen shots. And if it's making money, it's also a valuation of how much and multiply that over 2-2.5 months. It's just like a business, you won't pay more than 2X-2.5X what it makes. Now, a real business is valued at 2X-2.5X what it makes in a year. Instagram is not a real business.
Ask Vine 1M+ account owners. It's here today, gone tomorrow. You do not control it, it can be taken away by Instagram at any time for any reason. You do not control the followers, and if the account is deactivated or taken away you can't start another similar account with the same following and engagement within a few days, weeks or even months. So, anyone paying 1X, 2X or 3X what an IG account makes in 1 year is ignorant, a fool, and a bad business buyer, let alone a business person.
Now, others will say, "well I make $500 a month, so why would I sell for $1K or $2K"? My answer is don't! If you're making money keep the account. If you're selling it's because you have medical bills, your family member needs an operation, or you're lying about what you make.
No one sells a McDonald's, unless the place is going broke! Period!