Savings compared to disposables are really difficult to compare. There are so many different factors: which disposables you use, how many you use, which washable nappies you buy, are they second hand or new etc.
There are some suggestions here for a comparison. It may include some factors you’ve not considered!
You can join our local buying and selling group here to search for second hand options: https://www.facebook.com/groups/542628272452719/