Very roughly, brick/concrete pavers run around $3-5/SF for the pavers, stone from $5-10/SF (up to $50/SF for fancy marbles, granites, travertine, etc) depending on what kind of stone you are using and whether it is cut (pricier but flatter surface) or split (cheapest) and whether it is locally produced or not. Installation on good existing level subgrade materials would be in the ballpark of $10/SF, more if you are on clayey or organic soil or area needs more than superficiall levelling or drainage.
Ordinary in-ground fire pit from about $250 (dry fitted, ungrouted stone or stacked pavers) to $500-1000 for a grouted one. That does not include any kind of seating benches.
Fire pits are nice to sit around - basically just a contained campfire. If you intend to barbecue on it with any frequency or cook more than marshmallows and hot dogs, then you probably want a fireplace. However, unless you are a serious outdoor cook, the $1000-3000 for a fireplace is quite a hit compared to a couple hundred dollars for a portable metal barbecue or portable metal outdoor fireplace. Ditto for the fire pit - you can get a low metal fire pit about 30 inchers in diameter at box stores for about $50.