Let's see...here is my current "D&D" spending (all prices in USD):
$55 per year (~$5/month) on D&D Beyond Master Tier subscription (allows me to share unlocked content with players in my campaign, I can do this for 3 campaigns, up to 12 players per campaign, so I can share with 36 people)
$964 for purchasing everything on DDB. I didn't buy it all at once, but that is what the legendary bundle will cost you today, without a coupon or sale. If all that content was available as a subscription it might be worth it if I didn't already own it. 964 comes to about $80 per month, but I've bought content over a period of years. Much of that content is adventure content and many people are not going to use enough of the adventure content to make paying more than 5 to 10 per month on a subscription worth it if they can just instead buy the few books they want.
$12/month for my World Builder tier subscription with Forge-VTT. That gives me 2 GB of game data, 20 GB for assets library storage, 250 MB per file asset, no limit on number of worlds, any community modules or any assets bought from the Forge marketplace does not count towards storage asset library limit, save points, advanced invite management and user integration.
$50 my one-time purchase for Foundry license, which I host on Forge-VTT.
$10 per month to Patreon ($5 to Mr. Primate for DDB Importer for Foundry, allows for all monsters and characters to be imported into Foundry. Also support importing adventures, fully prepped, but there can be a quite a lag between when adventures are available in DDB and when they are fully complete for Foundry. $5 per month to Moulinette. This is a mod to Foundry to make it simple to create custom tiles among other things. The 5/month is primarily to get access to an ever growing library of artwork that integrates with the mod.
Total monthly digital D&D stuff subscription spend = $27 (rounded). But this price doesn't include the actual content.
If WotC could offer a subscription of $30/month that would give you access to all content on DDB plus the VTT tools, that would be an easy buy decisions for many people I think. If you already bought or choose to buy content rather than subscribe to it, then 15-20 per month makes sense for content sharing, character building and character sheets, and VTT. I think $50/year for everything would be the high end they could get away with, but I think 50 per month would price a lot of people out.
WotC's VTT would have to be really good to get me to pay an additional subscription or to end my Forge VTT subscription and only use the WotC VTT.