I created four scripts written in php. Scripts are used to import/export data between two databases (one database is on local machine and second one is on server as web shop).
Scripts are not basic import/export, because I had a lot of details on what to do with the data - here is just one example of complication: one database was on one language and second database was on second language.

I didn't give any price and I don't know what price should be for this four scripts -- what is their value?
I gave this script to my friend and theese scripts will be copied for more than one client.
Now, I should tell him about my price and I don't know how much. My only wish, I would like to be monthly paid.

Your opinions, thank you ;)

That's highly subjective. What might be a difficult and tedious job for you, can be an hour work for someone else. Your friend already has what he needs (your scripts), yet you haven't discussed payment terms? Next time, do this up front. How long did you work on this?

You are wright!
I worked about one month except sundays, about 4h per day, that's about 26 day * 4h = 104hours.
I'm not a professional in php, so that's why it is too long.

So how much is your time worth? Suppose your asking price is $10/hr, then you're talking about a $1000. If you want to get paid per month, that would be $80/month for a year, $40/month for two and so on. I assume your friend has expectations about what he has to pay. Does the above match that?

Ok, that's what I have in my mind. About 10-12$/h, and that would be around 85$/per month. I'll try to recommend this price and see response.
What about other clients, should I ignore that fact and focus only on my scripts?

Around *100$ per month, sorry

For other clients (duplicates of your script) where you basically have to do nothing I'd try to negotiate a lease fee. Depends on how much your friend is sellling this for. Say you can get $10/client/month. In the end it can add up nicely.

But that is just a fiction, wright?
What would be good arguments to get 10-12$/client/month?

Fiction, maybe. This is what you should have agreed upon before starting IMO. One good argument would be to be available for minor issues or (client-specific) updates.

I'm updated now so I will try not to do the same mistake again.
Thank you!!