Are there many steps to get the functionality I described as Sobipro?
Is it difficult combine both Filed Restriction and a membership & subscription plugin? What do you think?
Quite a long time ago since I had a look into SOBI v1 ... but I remember it was very special in regards of field-based payment and restriction. I can't remember another extension, where it worked in such a specific way. It was build to work like that, it has that workflow from the early days, AFAIK.
So, I don't think you can "copy" the exact workflow with two separate extensions. Maybe you could "mimic" it to a certain extend. It also depends on your structure and content types...
AFAIK, there is no similar "field-based" approach in other subscription extension. At least not with Seblod out-of-the-box.
One solution I know is "Emerald" and "Cobalt CCK", where you have a tighter integration on field level. i.e. a Subscription plan can restrict one or more fields in different tasks (submit, edit, view, display, etc.). You don't even need a usergroup/ACL. Although, you do
not have the SOBI behavior, where you can check the additonal field during submission... as an optional price addon. I think, this is quite unique in SOBI.
Back to Seblod... I don't think that you can get the "buy-this-field" feature with another Subscription extension. Even if it provides a plugin syntax. Most likely, the subscription software will display a restriction hint or placeholder... and you might also show a link to buy a certain subscription plan. Actually, that will redirect the user to a plan purchase page.
If you want multiple and independently restricted fields, you would need multiple plans. Or group certain premium fields in one plan. Or allow a certain usergroup/plan to access all premium fields.
Quite different to what SOBI does.