Monty gets a fee for presenting GW. I suspect that, plus spin off earnings for articles written for GW magazine, other magazines and newspapers and books related to his series on French, Italian and Round the World gardens are sufficient to pay for such a greenhouse without the Beeb having to pay a separate sum.
Either way, it's immaterial. AT made a fortune from his books on How To Be A Gardener which allowed him to buy a new house and garden which he'd always promised his wife would not be invaded by cameras.
GH took a punt and bought the land which allowed him to develop many gardens for GW and other series such as Paradise Gardens and The Ornamental Kitchen Gardens. Luckliy for him it worked and his fees from GW, articles and books helped pay for it. The difference was that GW was then produced by an independent company and sold to the Beeb.
With Monty's versions of GW, and the subsequent Greenacres fiasco with Toby, it's been a BBC team that produces and I still don't think they do it as well as the GH lot did nor as well as the outside company that produces Beechgrove.
I wish the Beeb would have the foresight and conviction to set up a national garden like Beechgrove which has the scope for various styles and sizes of garden within it plus trials beds and greenhouses and polytunnels and an acknowledged team of permanent gardeners who keep things going when the presenters are elsewhere doing their day job. I'm still waiting for Monty to introduce us to his team and give them credit.