Welcome to the forum!
Good question for the Ready Room, but since you asked...
First thing I would do is look at online markets and auction sites, and see what other people ask for the kit, and what the final selling prices are. If you don't find that particular kit, you can look at similar kits by the same manufacturer, or of similar scale, or of a similar subject, and so on. Even just running some web searches on your kit is a place to start.
Second, realize that prices on the secondary market are rather speculative than fixed like retail prices. In the end, the price will be what the seller and buyer agree on at the time. Also, be aware that a kit builder will prefer to pay a different price, usually lower, than a kit collector. And there are kit collectors, people who collect kits-no, not accumulate a stash with the intention of building-for things like box art, or kits they remember from their childhood, for rarity, and probably as many other reasons as collectors. A builder doesn't need a box, original decals, sealed bags, even instructions, as long as the important parts are included. If the kit is still in production, a builder might not even give one for sale a second glance. A collector will probably be willing to pay a higher price than the builder.
Those are my observations over years of building, buying, and watching sales on line and at shows. Others will probably have other observations of their own.