The three options are not variations on the same treatment. They work in fundamentally different ways, and understanding that difference explains almost every cost and longevity figure that follows.
A dental implant is a titanium post placed into the jawbone, which fuses with the bone over several months and then carries a crown. It replaces the root of the tooth, not just the visible part. Because it transmits chewing force into the bone the way a natural root does, the bone around it stays stimulated and does not recede.
A conventional bridge replaces the missing tooth with a false one suspended between the two neighbouring teeth. Those neighbouring teeth must be filed down, often substantially, so that crowns can be fitted over them to carry the bridge. Nothing is placed into the bone, so the bone beneath the gap continues to resorb. Note that this is a different treatment from an implant supported bridge, which is anchored to implants rather than to your own teeth and does not damage them.
A denture is a removable prosthesis that rests on the gum, held in place by suction, clasps or adhesive. It replaces the visible teeth only and sits on top of a jaw that keeps shrinking beneath it, which is why dentures need periodic relining to stay fitted. An implant supported denture is again a different treatment, clipping onto two to four implants for stability while remaining removable.
Everything below compares implants against conventional bridges and conventional removable dentures, because those are the three options patients are usually weighing against each other.