Wiltshire & Swindon History Centre (WSHC) has a nice selection of records relating to men who served in the Wiltshire Militia (Ref:1589/46) especially for substitutes. From 1757 onwards, those chosen by ballot were compelled to serve, but they could instead provide a substitute or pay £10 towards the provision of one:
In 1798, Robert NEWMAN, substitute for Joseph Butler HANKS in the supplementary militia “…hath left Mary his Wife and one Child, born in Wedlock, and under the Age of ten years…unable to support themselves”. The Overseers were ordered to pay Two Shillings weekly.
After a month serving as substitute for Thomas YEO in 1803, Joseph SMITH had “not been disapproved of and discharged by the Commanding Officer”. Presumably Thomas could not provide a substitute himself and as he was “not possessed of lands…” of a sufficient value (£500), it was ordered that the sum of five pounds, being “Half the current Price paid for a Volunteer” be found from either the parish Volunteer rate (if there was one) or the Poor rate.