Develop plugin cost

i found someone to designe a plugin extension but i don’t know how to calculate the designe cost

That’s a matter of negotiation between you and the designer. There are lots of possible ways to calculate cost, ranging from value (what is it worth to you) to amount of effort needed, either estimated in advance or based on time taken. Developers set widely varying hourly rates, depending on how hungry they are and where they live.

can you give an example you know from before as close estimation

I paid someone 100$ for a script that they got back to me in about an hour. I also sent someone 50$ for another one that they said took them 15 minutes to write.

For me the cost is minimal as all of them save me considerable time.

if my job will take about 35 hours what do you think as avarage

I have no idea… ask the plugin developer?
If I was billing you it would be 3500$