It depends on :
- a niche
- quality of your tweets
- daily activity on that account (how many posts you make, how many followers read and so on..)
You need to run several tests:
Test #1
1. send a couple of quality tweets with hot topic and good content
2. use short URL service to count clicks and Geo traffic
In 1 day, check out your stats (short URL) and see how many clicks you actually got.
Of course, not all the clicks will be generated by real followers.
Depending on the methods you've been using to generate followers, you can get 50 - 70 % fake clicks from bots.
It's approximately of course. But at least, this test will show you how responsive your followers can be.
Write down the clicks and traffic locations in a notepad and move on to the second test.
Test # 2
1. make a simple landing page and add opt-in form
2. send a couple of tweets (quality content) to your opt-in page
3. offer a free gift or something valuable for free in your niche
4. check out your email opt-in stats (how many of them joined your list)
This way you will see how your followers trust you, and if they are open for downloading free stuff from you.
Write down the opt-ins amount into your notepad and move on to the final test
Test # 3
1. make a good sale page
2. send a couple of tweets with a link to that sale page
3. try to sell them a product/service in your niche
Write down how many sales you got into your notepad.
After running these 3 simple tests you can answer your question - how valuable your 250K followers!
Hope you will find out the answer
Take care, bro.