I agree with Leptis, with a few more extra points to share

Most expensive: Surely Cruz. No matter it's their base pieces (DE weapon, DE torso/BD torso, good secret boots/BD wings), or their gems (Rage and Wind), or neck and rings, it's more expensive than other classes in all perspectives
Cheapest: imo it's surely SM. Their DE is kinda trash (except Con DE staff), which is both fortunate and unfortunate for this class. It's cheap to make a decent one, but even you want to spend a lot and make a top SM, but there's nothing much you can work on except BD torso and Con DE staff. Also since SM on this server focus mostly only on defense and forget about offense, you pretty much only need to gem UG colo and BD soul (or some BD heart if you want)
For other classes, whether cheap or not depend on whether you want to push to top, or just build a decent char.
Champion: it's quite cheap to just be decent. Hep Frame, Hep claw, colossus sword, and CA paw are all cheap yet very good pieces. However if you want to build a top champion, you'll go for DE, which increase the cost by a lot. Also due to they use similar gem as Cruz, gemming up isn't cheap either.
SS: Can be cheap to be made decent given that UG strike is the cheapest UG out their, you can build a UG4 SS with quite low cost (probably close to UG1 Cruz). However if you want to build a top SS, you might still want to go for DE gun, BD or DE torso, and spend a lot on BD gems to build up their defense.
Cler: if you just want to build a decent Cler all you need is levels because you need skill pts. You can do your job with just CA set. As long as you can buff/heal your teammates you're good to go. However if you want to build top Cler, you'll need to go for full unfused BD set, which costs $$$$ to make

Voy: i'd like to seperate this into Spr voy and Con voy
-Spr voy: Spr voy usually have low defense and HP, making them struggles a lot at HL. If you want to build a top Spr voy, you'd better to have very high level to help defense, and many BD gems (BD soul and heart) to help your PR and HP. Also you might need to use more pots and bless than others during pk (see Arabi). Or if you want to maximize your bolt damage, you can go full unfused BD set (see Eclipse). It can be quite costly, but it will have cute damage.
-Con voy: it's cheaper to make a decent one. With some nice PR robes and high Con items you can make one. They're quite good at CA, and if you have some spr items, they can also do good damage. Given some top items (high PR robes and Con DE dagger) and gems, you can carry a whole team in CA (see Sphere)