The filing fee of $10 for a caveat was less costly than the filing fee $15 for a full patent application. As stated by the USPTO: "In 1861, the fee for obtaining a full patent was $35, of which $15 was to be paid at the time of application and $20 when the patent was granted.
Filing a caveat was more economical than submitting a comprehensive patent application, which required a $15 filing fee. The USPTO explained in 1861 that the full patent application cost $35, with $15 paid upfront and $20 added when the patent was issued.