Good question, @pmario -and the way you put it calls for a somewhat nuanced answer.
In the field of PERSONAL productivity tools, there are umpteen free tools around that support all essential GTD functions- tho never in the same tool, it seems to me, unless with “some assembly (ha! usually a lot) required.” For a tool that integrates all in such a way as to facilitate seamless GTD workflows -AND enables a reasonable modicum of interop (i.e. dynamic dataflow) in some standard format (ideally: Markdown content serialised via RSS) to one’s groupware tools of choice [1]- i for one would happily pay $10/mo, as would many GTD aficionados, i believe.
[1] Now, as for GROUPWARE: that’s like another ballgame entirely. In that category, it hardly matters what i would consider reasonable; it’s a matter of what the market (i.e. companies) are willing to pay… And looking at the dominant players in this category (in no particular order: Asana, Jira, Monday.com, Trello… Maybe Evernote, tho here i’m not so clear [2]), pricing ranges from like $10 /month to $50 /month for “industrial strength” functionality, tho some offer most essential functionality for small teams in a free version (Trello, for example, from which my farm team has derived great value over the years, without ever paying a license fee). Again: optimisation for support of GTD workflows is not built-in to any of these products, but they are all more-or-less customisable (sometimes using plugin extensions arising out of a strong user community) to support at least some aspect(s) of GTD workflows.
[2] Considering all of the above, the field of products that bridges the two worlds of personal tools for GTD vs groupware is so narrow as to be (for me, anyway[3]) nonexistent. The one tool that i know to be pretty good for personal use (this much i know from personal experience of >10 ybp!) and also for team use (this much i know only by hearsay) is Evernote -making it perhaps the one to beat for TW5-GTD. That product, according to Developer’s pricing page is priced at $13.99 /user/month, at entry (i.e. 2-users) level.
[3] Reasonable as that pricing may be, i am not rushing to board that particular boat- nor any of the aforementioned, even if they too should offer some such thing- because i know how much time & work would be involved in (re)learning “the Evernote way” and optimising it for GTD workflows… Let alone selling my colleagues (none of whom is really into GTD, AFAIK) on the idea of adapting to my adopted system! We are as a team more-or-less comfortable using Trello… But that’s a lousy tool for personal GTD (especially for those of us who value the agility of a tool like TW5).
No: the more promising approach from my perspective would be to build some teamwork functionality into TW5, somehow -a pretty tough row to hoe, i gather, from everything i’ve been able to gather here (and i’ve been watching this TW community for signs of any groupware that might emerge for a good few years already, NB). Still: that is a most noble cause, and i applaud all TW developers working on solving the many problems involved in getting some multi-user/editor capable instance operable in the cloud.
MEANWHILE: the much easier thing i’m expecting to see first should be a really solid Edition of TW5 for GTD -solid enough that it should engage a significant user community of its own, culled from the huge community of GTD mavens, most of whom are of course not members of this TW community. In fact, i daresay that most serious GTD’ers might well have an allergic reaction to TW, because TW is sooo very conducive to that tweaking-the-software tendency that is the very antithesis of what GTD is all about.
My €0.02, FWIW.
/walt
