It could have been a single Euro and it would still be outrageous. I'm not aware of any country that did
not raise the fees after introducing them. In fact, universities in Germany were generally free to set the fees as low as they want and nearly every single one of them set them to the maximum allowed by the bill.
It's a matter of principles. Introducing tuition fees at all means you're abandoning the idea of universities being open to everyone. Sure, you can subsidize those unable to afford the fees, but by adding such hurdles you effectively raise the barrier to entry.
And the claim that this would bring extra money is pretty naive. The idea was to shift the burden of maintaining universities from the general public (via taxes) to the students (via tuition fees) in order to be able to lower (or avoid raising) the public subsidies. It would have allowed the government to reduce the subsidies arbitrarily because the universities could still make up for the difference by raising the tuition fees.
Thinking that the money was needed is also pretty naive. The bill required universities to spend the money on improving the quality of education (e.g. more student tutors, better equipment, etc). The university I studied at spent the money on new buildings, including one purely administrative building that replaced the only major car park available to students in the vicinity of the main campus. But the vast majority of the money was never spent -- they only started shoving money into these building projects because they were threatened with being sued over keeping the money in the bank (to attract interest).
Oh, and at my university at least (University of Cologne), the invoices students received were accompanied by free advertising for the NRW.Bank (effectively a government-owned private company), offering a "special" loan with a slightly lower interest than other banks. The process for signing up was also very streamlined: they'd pay the tuition fees for you, you just pay the regular baseline fee (~280 EUR back then) as before and you could pretend the tuition fees don't exist until you've graduated and need to pay the loan (and accumulated interest) back. Very thoughtful of them, but also very devious.
It also didn't help that our principal tried to dispel the protesters by saying that every student could afford the tuition if only they drank one less beer per day.
But the primary reason student organizations found the tuition fees so appalling is the trend happening at the time: the switch to the BA/MA system, the reduction of high school from 13 to 12 years, the "Eliteuniversität" programme (which incentivised further shifting money from niche subjects to the more attractive ones (basically anything likely to receive outside funding from major corporations)) and the general "Verschulung" of higher education to make university degrees more useful for HR and less about real education or science, effectively turning universities into US-style diploma mills.
The original German ideal of university (compared to the more business-oriented Fachhochschule and the traditional IHK job training system or the craft Geselle/Meister systems) is the studium generale or studium universale. In other words: the renaissance man approach to science, striving to become a polymath by educating yourself to achieve wisdom and become a more informed citizen. That education should be open and available to all who seek it out.
The origin of this ideal is generally ascribed to Humboldt. The general trend of adapting the more "open" university system to the "needs of the economy" (this was a major selling point for the BA/MA switch back then) and thus further blurring the distinction between university and other forms of (job-related) higher education stands directly opposed to this ideal. Whether you think this ideal has any worth or not (or even whether it is remotely realistic or not), it was a major source of conflict during the protests.