Thank you in advance!
How many megabytes of features are you storing? Its easy to estimate your costs (many assumptions including credit costs are pro-rated and not including any credits built into your subscription):
Megabytes of Feature Storage / 10MB * 2.4 credits * 0.1 $perCredit * 12months = $perYear
1MB = $0.29
100MB = $28.80
1000MB = $288.00
Once you know your costs you could check out other commercial providers who can host features and make services available to try and find a better deal. But that would have to be by WFS and not Esri REST API I suppose.
If you are getting up into the 10's of gigabytes then the dedicated feature store options make more sense pricewise, as they include 1 terabyte of feature storage in the price.
There are other ways to reduce the cost as well. Do all the attribute fields need to be included? Are the length of text fields optimised? Could text strings be replaced by codes? Possible to optimise the data to reduce the number of vertices etc.?