I have found that a good place to start when looking for the sale value of a Beanie Baby is to check out the market demand on sites like eBay and Amazon.
This way you can compare your toys directly to people that also have the second hand toys, and see how much they are selling them for. The official TY collectors website recommends that this is the most accurate method of seeing how much each individual toy is worth, as their values change so quickly.
Just make sure that the listings you are looking at are from no more than 60 days ago.
eBay have a 'sold listings' page, so you can check how much each listing has actually sold for, rather than the asking price.
I advise against trusting any value guides that are printed, as often the prices that are listed in these are very high. I is unlikely you will get anywhere near this price for your toys, even if they are in excellent condition.
I have also found that you can get so much more for the original toys if they have the swing tag still attached to them. If they don't have this then I'm afraid you are only ever going to get about 50% of what the toy is worth. For a collector, having the tag still attached is very important and will make the toy so much more desirable to them.
Other things factors that will affect the sell value are the condition of the toy and how rare they are. Obviously the rarer the toy and the better condition it is, then the more money you can get for it.