It’s a little hard to give exact costs for both instances as a whole, because they share infrastructure (i.e. shared email host is one cost across both) but the server itself for Lemmy is just under 50 euros a month, plus image storage at another 15/16 USD per month (with around 2TB of images). The smaller piefed server is more in the region of 25 euros a month, with images more in the 5/10USD range. Then domain on top of that. It’s not “expensive” as such, but it is time-heavy.