Actually not very expensive. I'd go as far as saying it costs less than a standard scaper tak or community tank. The plants I got from other aquarists for free, mostly. There is only one I paid money for in there right now. 3,99€.
Sand cost me about 5,-. Rocks are almost exclusively collected from the river, the leaves and
alder cones that are in right now I had to buy because I ran out of self-collected ones from last autmn. Only thing moderately expensive was the
driftwood, which I altogether paid 30€ for. Other "expensive" stuff pays off soon as one usually only buys certain things once. I'm going to get a small RO unit soon, which will pay off within three months according to my calculation.
Monthly upkeep is at about 5,- as soon as the RO unit is in place, right now I have to mix in distilled water because of super high
nitrates from the tap, which pushes the costs to 25€ a month.
But alltogether... The decorations and plants that are in there right now... 40 bucks?
The really tricky part: Weeks and months of Research. For a biotope tank you research everything around a certain species of fish or a biotope type and you don't stop. It goes on and on. Substrates, plants, fish, microorganisms, ecology, seasons,
water parameters, water chemistry... and so on.