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Some clients, let's say 5 <= x <= 10, have to send information wirelessly to a server. Transmission does not need to be reliable (ACK).
The range would be between 50 and 100m, and may include soft obstacles like trees, cars, etc, but not major ones like whole buildings or concrete walls.
Also, as I plan to build about 20 clients, the price should be as low as possible.
Thoughts?
Depends on how much data you're sending, what packet structure you want to use, whether you want to spin your own PCBs or use a plug-in module, whether you need a low-power solution, etc..
For that distance, probably a sub-1GHz solution would work best.
I've had good results with the TI range of Value Line transceivers as they offload all the packet-handling, sync-words, CRC etc. from the microcontroller. The modules also handle addressing, and have a "listen before talking" functionality - both of which could be useful with your physical architecture.
Down sides:
RF433 modules are about as cheap as you get. You can get a tx/rx pair(separate modules) for 1-2 USD on ebay. Just search for "433mhz module". 100m or longer range should be possible with a proper antenna - a wire of the correct length. Use wavelength / 4, for 433MHz that is ~173mm.
The modules just communicate over a serial interface and they send the data straight to the air. But for better transmission the 1's and 0's need to be balanced and a "training preamble" before a data transmit balances the receiver for better noise immunity. Use a library such as virtualwire to take care of data decoding/encoding.
The common cheap transmitters on ebay allow you to supply a higher voltage to the module for increased transmission power.
Alternatively if you want transmitter and receiver in the same module you could go for nrf24l01 based modules. I haven't yet used them myself but they are dirt cheap as well. They operate at 2.4Ghz using a proprietary protocol with retransmit etc built-in. The modules have way more pins than the cheap RF433 modules, but you can also find arduino libraries to interface with them. The cheap kind use an antenna etched straight on to the print board. According to this article the open air range is 50-80m depending in the data payload in a transmission.
