What we do know about D&D Beyond is the total number of sales, since all purchases have a sequential order number.
My first order (on 2017-08-15) was number 00009 (what can I say, I'm an early adopter!) and my most recent one (on 2023-09-22) was 12567395. Looking back over my history for the last six years, I can provide order numbers from roughly the same time each year, as follows:
2017-08-15: 00009
2018-07-23: 265845
2019-08-22: 1240140
2020-08-17: 3107000
2021-07-16: 5565731
2022-08-17: 9157194
2023-08-03: 12131266
From this we can estimate the number of individual sales for each year of D&D Beyond's existence, as follows:
Year 1: 266k
Year 2: 974k
Year 3: 1.87m
Year 4: 2.46m
Year 5: 3.60m
Year 6: 2.97m
We can see from this that the first five years had significant growth each year, but the volume of sales dropped slightly in 2022/2023.
This doesn't give us any information about the value of those sales, but we can do some very rough estimates. Subscriptions are either $55 or $26 annually. Prices for individual items on DDB vary from about $5 for dice to $100 for The Deck of Many Things physical+digital bundle, with the initial Legendary Bundle price of $280 being an outlier. There are a lot of variables here. It's possible to buy multiple books at once. It's possible to buy individual components (monster, class options). DDB has frequent sales with significant discounts. But, for the purpose of this wild estimation, let's assume that a typical transaction is the purchase of one book at an average price of $30.
What do these VERY wild assumptions give us? Multiplying $30 by the number of sales per year gives the following dollar amounts:
Year 1: $8m
Year 2: $29m
Year 3: $56m
Year 4: $74m
Year 5: $108m
Year 6: $89m