Best is probably one of the auction result databases such as Pro Coin Archives but you may have to pay a subscription fee. If you save a few hundred dollars in your Taler purchases, you will still come out ahead.
I use acsearch and am very happy with it. It also will cost you (about $90/year) but it will tell you how much comparative pieces have brought at auctions going back about 20 years.
I agree with what talerman and Eduard already offered. For me, the early research starts with my 8 year old Krause "German Coins version 1501 - present" as a start to get a feel for relative scarcity. Prices might not be on target but it certainly gives a sense of the general ballpark. Next I use acsearch as a online reference for the number of times the piece comes to auction. If it is a more common piece in a scarce grade, Heritage history can help also. I do not have a current subscription to acsearch, so if I need to get the most recent auction hammer prices, I use sixbid to hunt and peck through the most recent completed auctions to get a baseline. Of course, I have learned to throw all this out the window if it is a choice piece or if it is being auctioned by a premier house like Kunker....I have been the underbidder more times than I can count in recent years.
The NGC world price guide can also be a reference; I suspect that it is closely correlated with Krause values. https://www.ngccoin.com/price-guide/world/