From what I've understood from articles over the years neither DS9, Voyager nor Enterprise were cheaper to produce at least not until Enterprise's final season.
DS9 had a lot of expensive start-up costs, due to the nature of it having to build all those unique sets and costumes, but
in theory Voyager was always conceived to be a cheapy, simply migrating assets over from TNG with only surface-level changes to the design and layout of them (the sets are, for example, mostly easily identenfiable as the same, except for the bridge itself which was a new-build), with a cast whose pay checks were considerably less than their TNG counterparts were getting after seven successful seasons of renewals. For various unforeseen reasons, though, there was a budget blow-out due to reshoots and things like that, and Voyager's budget ended up running into the red for the best part of the first couple of seasons as a consequence of the ball being dropped early on.
But on paper, the idea was that they could have their cake and eat it too (ie, make a 'successor series' to TNG, without it being as expensive as TNG was getting to be at that point).