I voted $5. It isn't that I don't think Dominion is better than $5 per expansion; I'm just being realistic. Whenever I buy digital games, I generally get them on sale or for very low prices. Maybe just because I grew up when everything you could buy was physical for the most part, I don't see as much value in digital items. If I lose computer access, suddenly I've got nothing, whereas if I have a physical game, well, I still own that. I've bought all the physical Dominion sets too, most of them at full price, and I thought they were worth that even though I play them less. Plus, I never had much faith Goko/whoever would still be around in a few years, so I waited for them to lower the price they were initially asking for.
For pointless analysis, I'll look at two other games I own digitally that I love almost as much as Dominion. I got Dark Souls for $5 and have played over 300 hours of it so far. I got Plants vs Zombies for $1 on two different platforms and have played probably 100+ hours across various devices, plus my wife has played it almost as many. That is a ton of mileage out of a tiny cost, much less than I paid for Dominion Online. I've probably played more Dominion than that, but if we go by those dollar amounts the whole thing would still only be 'worth' $15 tops. Obviously these games are incompatible, but I think a lot of PC and mobile gamers have this experience.
So I'm saying $5 per expansion is fair, but I'm just saying that because I like Dominion as much as I do. To the average computer game player, it probably seems steep since I imagine they tend to buy things on the cheap as well. That said, I have played Dominion online enough to probably justify having spent hundreds of dollars on it (but they don't charge monthly so I guess they didn't get all that from me).