I am an old chap who has been cruising the internet since it started, and dial-up bulletin boards before that. Never have I seen such an amazing question posted, and I love it.
There is not a phone number permanently associated with any sim. Each sim has a 20 digit serial number that is registered on a network to link it with a phone number, which always belongs to the network. That's why you can always get your number back if your phone is lost or stolen, you just tell the network your new sim serial and they link it to the number.
When a pre-pay sim is inactive for six months the network will recycle the number, and if a contract is not renewed the number goes back into the pot. During my unhappy time working for Vodafone a fairly common complaint was that people would get calls from someone who knew the previous user of the number. Usually those calls were entirely harmless, but sometimes not. If that happens to you, kick up a fuss and you will get a free number change, which takes only seconds.
Short answer to the OP, your number is worth nothing. My usual number valuation fee is