Let's not pretend software developers are paid exceptionally well in the Netherlands. Somehow it's a skill that's way undervalued here, compared to e.g. middle-management. That said, a senior dev with an academic background (ie: if you can do more than just programming but also have the background to acquire and use complex domain-specific knowledge) you can easily make over $6000 gross a month. The cost of hiring you will be almost double that for the employer.
You're not going to make that kind of money by working projects for a consultancy firm though, the margins they are making on your skills are ridiculous.
You can easily make over 100k a year as a dev in NL if you're self employed, assuming you charge 60 an hour (about half the rate a consultancy firm charges customers) and work 1800 hours a year. That way, you have no benefits, no insurance, and no income when you're between projects, but an income that your English speaking peers at least don't laugh as hard at.
I don't think the wages are all that bad considering the generous benefits. What's really killing is the cost of living here.
What you charge your customer as self-employed dev can't be compared to what an employee makes. The rule of thumb I heard is that you should consider about a third to be "net income". The rest is taxes, investments and various costs (including insurance and retirement).
So a self-employed dev making 100k would be comparable to an employee making 30-40k. It's a very rough estimate; it's probably a bit better than that.
I'm self-employed and have about €120k revenue per year (working 4 days a week), and I think my taxable income ended up below €60k last year.
You're probably referring to health insurance, but that isn't a major problem. Health insurance is commercial, but regulated.
Employees are insured for unemployment, disability etc. Your employer is insured to cover your wages if you're sick. If you're self employed, you either don't get paid for hours not spent working, or you have to buy similar insurances commercially. You will also need insurance for liability and probably for legal costs. If you're good at your job and good at selling yourself, you probably make more than an employee would, but the 60 an hour are not pure profit.
You're not going to make that kind of money by working projects for a consultancy firm though, the margins they are making on your skills are ridiculous.