Essentially you are paying for the quality of the bulb itself and circuitry. The Sensibulb uses Japanese made LED's (this is the same bulb Nikon uses in it's very expensive medical imaging equipment) that cost about six times what the Marinebeam Seoul Semiconductor Korean made bulbs do.
I'm not sure that if by "bulbs" you mean "emitters," but just in case: SSC LEDs/emitters aren't necessarily inferior. Like any other LED emitter: They need to be driven and heat-sinked properly. Over-drive and/or under-heat-sink them and they'll die.
LEDs are semi-conductors. Heat is a semi-conductor's enemy. To get LEDs to emit a bright light they must unfortunately be driven to the point that they generate self-destructive heat. The only way to save them is for there to be adequate heat-sinking to draw the heat away. You can buy relatively inexpensive LED flashlights these days that way
out-perform a "traditional" flashlight--even a high-end one from such as MagLight, but you can not
obtain a LED replacement bulb for "old tech" flashlights that will make them perform as well, because there's no way to design a drop-in replacement bulb with the heat-sinking necessary top allow you to drive the emitter hard enough to make it perform well.
This should tell you something about the likely longevity of (most?) replacement navigation "bulbs."