If you look on sites like Yahoo Auctions and Mercari, you can find at least one Wing of Madoola sample cartridge for sale at any given time, typically for around $200-300. These cartridges were used as prerelease samples, and were given to retailers as well as companies making strategy guides. You may be curious whether the cartridges are any different from the retail version, and if they're worth the premium over a standard cartridge (which costs $15-$25 as of this writing). Let's investigate the differences (some 6502 assembly knowledge is necessary here).
The only difference in the PRG (program) ROM has to do with the game initialization code. The Wing of Madoola has 2 ways to initialize a new game, which I have named "startNewGame" and "startMaxxedOutGame". startNewGame sets the player's health, max health, and max magic to 1000, and starts the player at stage 1. Meanwhile, startMaxxedOutGame sets the player's health, max health, and max magic to 9990, and lets the player continue up to stage 16. It also initializes the regular sword's level to 4 (which gets clamped down to level 3 by the weapon damage code), and all other weapons to level 3.
Here's the first few lines of code that the retail version runs after the player presses start on the title screen:
ROM:A982 LDA joy2 ROM:A984 NOP ROM:A985 NOP ROM:A986 LDA joy1 ROM:A988 AND #$20 ; was select pressed? (continue code) ROM:A98A BEQ startNewGame ; no? start new game
Obviously, the NOPs look kind of suspicious. Let's compare that to the sample version's code:
ROM:A982 LDA joy2 ; start a maxxed out game if ROM:A984 BMI startMaxxedOutGame ; A was pressed on controller 2 ROM:A986 LDA joy1 ROM:A988 AND #$20 ; was select pressed? (continue code) ROM:A98A BEQ startNewGame ; no? start new game
As you can see, the retail version has a cheat code patched out of it. Clever readers may realize that since the only difference between the sample and retail ROMs is those two bytes, it's possible to use a Game Genie to restore the cheat code's functionality in the retail game. The two codes you'll need to use are AUEZKP and IOEZSO. Alternatively, you can invert the condition so the cheat code will always trigger as long as A ISN'T being pressed on the second controller by using AOEZKP instead of AUEZKP.
The fact that the cheat code was patched out rather than the programmers removing it from the source code and reassembling the game indicates some sort of paranoia to me. My guess is that Sunsoft playtested the game with the cheat code enabled. Once the game was fully tested, they replaced it with NOPs to make sure that the assembly process didn't introduce any new bugs to their "perfect" build.
The only difference here has to do with the "Keyword" screen on Stage 8. Sunsoft ran a contest where if you mailed in a postcard with the keyword (ねこだよ〜ん), you could win a free cassette tape with arranged music from the game. Obviously, having a version of the game that allowed the player to skip directly to Stage 8 would put this contest in jeopardy, so their solution was to erase the keyword tiles from the CHR ROM:
Given that the only changes made in the sample version were adding a cheat code (which can be re-enabled in the retail version using a Game Genie) and to try to prevent players from cheating at a contest that ended in March 1987, I don't think that the sample cartridge is worth the extra money unless you collect sample cartridges or something.