I'd forgotten that Microsoft has killed Foundation (2013 will be the last version), so there is no longer a free version of Sharepoint. If you need it, you have to pay for it. I'd forgotten this even though I actually posted it on ML a year ago (I think my memory is going in my old age).
Add that to the fact that it appears that migrating from on-premise to online is not a simple task (compared with say Exchange), and I'm coming round to thinking it would be foolish to further invest in Sharepoint Foundation 2013 and we're better off moving to Sharepoint online asap. It will mean extra cost in the short-term, because we'll have to buy a load of O365 subscriptions, but less cost in the long-term (as eventually we will have to migrate from on-premise to online since Foundation is the only product that makes financial sense on-premise and Foundation has been killed).