This is a much harder question than it sounds and it depends on a number of factors. It is important to remember that there is a food chain in buying and selling antiques.
At one end a general dealer might buy something he thinks is medical with potential value from a car boot seller who does not know what it is, for say £10. Next if the general dealer turned out to be correct he might sell to a specialist medical dealer for £100. At the top end the specialist dealer might sell the same piece to a specialist collector for £1000. This example of the price increasing by an order of magnitude as it rises through the food chain is more common than you might imagine.
The internet makes information available about prices and specialist vendors in a way which increasingly allows dealers to be bypassed. The other factor which affects price considerably is demand. In a market place where availability is limited, it only takes two or three collectors competing with each other to see prices in that area double over relatively short time frames.
Likewise established prices can fluctuate considerably in response to availability. 10 years ago the cost of spring lancets and scarificators was over twice their current price after these items were made increasingly available to the market from Eastern Europe via the internet.