Currently, the Budgeting feature for products and services is unavailable. You can only subdivide it into Customers, Class, and Locations.
I recommend pulling up and customizing the Product/ Service Summary report to check the monthly sales. Then, export it to Excel and enter the budget so you can compare the sales and estimated budget. You can also check for the third-party apps for this feature.
Here's how to customize your Product/ Service Summary report:
Go to the Reports menu and enter the Product/ Service Summary report in the search box.
Click Customize button, then go to the Rows/ Column section.
Click on the Columns box and choose Months.
Select Run report.
I added a screenshot for your reference.
To check out for a third-party app that can have the feature that you need, here's our website page for that: https://intuit.me/30jA5se
Furthermore, I'm sharing this article to help you manage your reports in QuickBooks Online (QBO): Run reports in QuickBooks Online. This also contains other sub-topics and articles such as the popular reports in QBO.
Should you need anything else in QuickBooks Online, please comment below. I'd be glad to help. Have a great day ahead.