I admitedly have no proof of how much each unit costs, but if they are the type of thing that can be mass manufactured by robotics, and the only people needed are those doing quality control at the end, and those that are driving in the raw materials. Even at German wages, wage costs aren't exactly going to be sky high. I would not take long to check each filter, so lettas assum these guys are unrealisticaly expencive, and they take a £1 contribution from each filter on the way out. That laves £9 for the other materials. Plastic will cost mebe another 50p at bulk buying cost, leaving £8.50 for the relatively small amounts of metal in the product, and as contributions towards overheads.
Granted these are all hearsay, but if you think about economies of scale and robotical production lines, it isn't unrealistic to make the assumption that each unit costs £10-15 to make
Take another Greman manufacturing company Juwel Aquarium. I've visuited their factory, and I can tell you that they simply load the glass and silicone into their machines, and then the robots take over..Out comes completed tanks with the pump boxes already installed. One person per line then QC checks each tank, then they are sent to packaging where another 1 person adds the pump, filter sponges, lid with light pre-installed e.t.c. and robots take over again to complete packaging. A realy impressive operation, but there are as close to no labor costs in the process as is practicaly possible.
The techniques that Juwel have used for a few years now could easily be applied to Ehiems filters, and I'd be suppried if a company as big as Ehiem haven't made the switch to robotics already
All the best
Rabbut
at the end of the day, see what suits your price band, if you can comfortably afford an eheim then it would be good to do so, if tetra are more in your bracket then there a reasonable alternative 
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