Price Observations For Dutch Coins

After almost one year browsing for Dutch coins on ebay, US auction sites, European auction sites, American coin dealers and Dutch coin dealers, I have come to the conclusion that PRICE very much depends WHERE you buy a particular coin, rather than WHAT the coins is.
For instance, modern Dutch coins (post WW-2) in MS condition are priced much higher in North American than with Dutch dealers. I suppose this is because the Dutch simply have much more inventory. The same goes for key dates. Although often, you cannot even find a certain key date in the US or Canada.
Secondly, classic Dutch coins from the 19th Century up to WW-I can typically be obtained for a better price in the US, especially on auction websites. I am not sure why this is, but MS coins from this era are priced extremely high by Dutch dealers. However, if you need a key date, you just have to pay up. Another problem with Dutch dealers is that their classic coins are often cleaned, which I think is a shame. Modern coins do not seem to have this issue.
Things change again however if we move back in time to the Dutch Republic (1572-1795). I have found that you can get a better deal on coins such as Ducatons, silver Ducats, etc. on European auction website than either their US equivalents or any coin dealer anywhere. This probably has to do - once again - with supply. What does seem mindboggling though are the astronomical prices people pay for slabbed Ducatons and Ducat on auction websites like HA or SB. For some reason, a coin being slabbed often means that the "overpayment" reaches triple digits.