There may have been some confusion in the previous reply as one of the screenshots referred to user quota, but for client credentials (which is the flow where machine to machine quota would apply) there’s no refresh token currently issued so the second question does not apply.
In relation to your first question, the shorter the lifetime, technically the more frequently you would have to obtain a new machine to machine token (so costs would indeed be higher) because it would be expected that more tokens would need to be issued.
Also have in mind that although machine to machine is strictly related to client credentials (not all client credentials flows count for that quota as mentioned at this other reply. Spoiler: an access token issued with client credentials for the Auth0 Management API won’t count for example.