If you want to get paid a lot more and you have the right skills the City or Silicon Valley are good options (assuming you are allowed to move there).
It doesn’t need to be a huge tech hub at all. Luckily English resembles Dutch so most Dutch speak it well enough to survive a job abroad.
I know very well paid developers in NL and very poorly paid ones, it is mostly a matter of knowing what you are worth and refusing to charge less than that. You might find 'your spot' taken by a skilled immigrant but that's a relatively small chance.
I'd hate to have to find a developer job in Spain or Portugal judging by the number of people from there that have moved to either NL or DE. Anything East of the German-Polish border is going to pay a lot worse unless you are willing to move to Finland where there are pockets of start-ups with reasonable compensation.
Dutch developers moving abroad to increase their salary is not a trend as far as I can see.
I know well paid developers as well, some niches pay pretty well. SAP for example. Or simply people that have domain knowledge that's irreplaceable and know how to negotiate. But something average like C#, RoR or Node development doesn't really pay well in The Netherlands. It's a decent salary but nothing to brag about.
My take on this: it's so easy for Polish developers to move to and find a job in Western Europe, that local companies have to pay salary that provides at least a roughly comparable living standard (vs the West)- as otherwise talented people would just leave. For "suits", on the other hand, there's far less opportunities abroad, so their wages don't need to track Western standards so much.
This effect is even stronger in Ukraine, where a teacher will be paid $300 (per month) while a dev will make $3000.