This answer is made from various comments on the question (often found in any request to get to a cheap place).
The distance from Amsterdam to Paris is approximately 500 km, on the main roads. But since you'll want a good bike route, you can add a few miles, so let's assume it's about 600 km, about 375 miles.
For those who are experienced cyclists and know how much they can do in a day, it will be easy to determine how long it will take. It is relatively flat, less and less when you go further south.
Google Maps gives a height indication of 140 meters, 460 feet.
It will drop almost to the level at times, but it is not close to the heights you will have to do on high hills or mountains.
One thing you may have noticed is that there will be winds, which often blow from the south and can be strong at any time of the year.
For those with less experience, by taking distances of 60 km per day, it will take 10 days to get to Paris, and that is a distance that almost everyone with cycling experience can handle, new real cyclists may have to go further. in the distances. On a tight bike with the wind behind, 100km days are very possible.
If you cycle outside of the main holiday season, you don't have to make reservations and you can adjust your daily distances to how you feel. And you will be close to the railway lines where you can take the train with your bike if it turns out you don't like the experience.
Now about the costs. If you own a bike located in the Netherlands, it will only cost you the nights and food that you would generally spend money on during the holidays.
If you don't have a bike like that, but have a few weeks (and / or local friends) to find a second-hand bike, based on your needs, you can often find one for relatively little money. (I have seen samples of around € 25 for bikes that are good enough to undertake such a journey, but you will need time to find such a deal, don't expect to find a good cheap bike in a day or two.)
Renting a bicycle is also an option if you intend to return to Amsterdam, it is possible to return by train, without reservation, but you will have to change trains several times. (I don't think fast trains carry many bicycles, if there are any.)
As far as I know, there are still no "one-way rentals" for this distance. There are organized tours that will transport you (and your bike) back to the start, but they will be more expensive and less free to select your own route. Some of those tours will provide you with a bike to use, it may not be the best bike you have ever used, but it may be attractive to some people.
There are many long-distance bike routes, this site has many of those across Europe, as well as links to national organizations that will have even more routes. The search for & # 39; long-distance bike routes & # 39; You will find many more (as well as the organized hotel and the bike tours).
Hiking / walking.
There are many long-distance routes, often in a combination of several, that will cover from Amsterdam to Paris.
It's still about 500/600 km, 310 to 375 miles, or much more than less direct but more enjoyable trails to hike. This is quite a lot for most people in a short period of time. But then again, if you are experienced, you will know your daily distance and the elevation level will remain the same as for cyclists, 140 meters or approximately 460 feet.
Get a good guide to the long-distance routes you want to use, so you can find information on places to sleep near your route. (Hotel search sites will work, but you may need to find places that aren't on those sites, in case you're in an area where hotels are scarce.)
As with long-distance bike routes, there are websites for long-distance walking routes. Wikipedia has a few for international and national long distance in the target area (and beyond). Again, a search, such as long-distance walking and a country or route, will give you more information. The long-distance walking routes on those Wikipedia pages often have brochures for the route and additional information. But more and more information is found online as well or instead.
Unlike the bus (or any public transport) that travels by bike or on your own feet, it allows you to see the country you are traveling in, visit the places where you spend the night, maybe even make a stop or two in tourist spots, if you are willing to leave your (bicycle and) equipment unattended or stay close enough to walk or ride a bicycle without equipment.
Camping will work for both biking and walking, but it will increase the load you need to carry and the preparations and (air) luggage you need to carry. Hotels, hostels, B & B & # 39; s, airbnb and even hot showers (places for people on bicycles) and the like for people who walk, will be easier, since most of the time you just need your sleeping bag.
Many people will combine walking the best parts of the routes with public transportation for the rest, which will probably be much more expensive than the cheapest long-distance routes for the entire distance from Amsterdam / Paris.
While this is a cheap way to travel, people often spend more on places to stay and food, and it will certainly take much longer, several days, or even several weeks to walk and not go extreme distances each day. Actually, it is not a way to get to places, but a way to pass time and get somewhere almost as an afterthought.