I'd gladly pay for an eSignature API that has pay-as-you-go pricing or a good entry-level plan. Products like HelloSign or DocuSign have high monthly costs associated with API usage.
I was going to suggest Signable [1] which I have used in the past, but it looks like their API is only available on a monthly plan (I'm sure it was available on PAYG before) - but it is cheaper than the 2 you mentioned.
[1] https://www.signable.co.uk/plans/