10k per month is probably a reasonable number if the dev lives in US. That's like $2.5 per month. Assuming $5000 cost of living / month, only need 2000 paid users. But if the dev wants to have some savings and support their family well, probably need 4000 paid users.

Logarithmic decay of price in Sats is a good idea because the value of Sats against fiat should increase overtime.