Well, not being a scripter, me saying I don't think a script is needed is sort of useless, but I don't think a script is needed. Also, not using XP, I can't say how an event would be done, nessecarily. All that being said, I'd use a mini event system.
Page One
Text: "Hello, I exchange your Dragon Medals for Prizes!"
If Read How It's Done
Text: "Here's how this event works!"
If cash in medals
Shop Processing
Sword 10m
Shield 20m
Helmet 30m
Pet Were-Hamster 35m
And so on...
Or if you want the NPC just to hand out things in ascending order of cost, make several pages with SelfSwitches or Numbered Switches that switch to the next page when one item is paid for, and each page has a shop process where only the current "tier" of item is available.
Page One
Text: "Hello, I exchange your Dragon Medals for Prizes!"
Show Choices: Read how it's Done, Cash in Medals
If Read How It's Done
Text: "Here's how this event works!"
If cash in medals
Text:"This is what I have available!"
Shop Processing
Sword 10m
Self Switch A (or switch #x) on
Page Two
If SelfSwitch A (or switch #x) is ON
Text: "Hello, I exchange your Dragon Medals for Prizes!"
If Read How It's Done
Text: "Here's how this event works!"
If cash in medals
Shop Processing
Shield 20m
(If using Numbered Switches, turn switch #x OFF)
Self Switch B (or switch #y) on
Page Three
If SelfSwitch B (or switch #y) is ON
Text: "Hello, I exchange your Dragon Medals for Prizes!"
If Read How It's Done
Text: "Here's how this event works!"
If cash in medals
Shop Processing
Helmet 30m
(If using Numbered Switches, turn switch #y OFF)
Self Switch C (or switch #Z) on
And so on. At least, that's how you could do it in VX, so... I hope it helps.