What its worth monetarily does depend on a number of factors, such as the condition of it and what someone would want to pay for it. Please click on the following search. You will find that card on some of the links.
1990 Donruss Nolan Ryan 300th Win Baseball Card eBay - Google Search
I wish that we had a collectible card expert on this site. But unfortunately for right now, we don't. These kinds of questions come up fairly frequently here. You might want to put the two words, "baseball cards" or "baseball card" into the search rectangle at the top of this page and you will find lots of links to questions that have been about the value of baseball cards.
Also, when looking for the value of collectible cards, please keep in mind that the condition of the card makes a huge difference in how much it might be worth. Card collectors get really picky about the conditions of the cards when determining a possible value for them. So, it really would be best if an expert could be viewing the card in person.
Other resources that you might want to try would be a store that sells collectible cards, a bookstore that has books on collectible cards and also, a public library.
Here are also some other links that might be helpful to you:
Baseball Card | Value Price Guide | Selling Cards Worth Baseball Card Value Price Guide based on prices realized in auctions. Email alerts on pending eBay auctions at no cost to you. Basketball, Football, Hockey and Boxing Card Price Guide. Track Card Values like a Stock Portfolio. VintageCardPrices.com
There are also links on the bottom of this page to questions that have been similar to yours.
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