I wanna create queue management system (for bank, hostpital…) Something like that 
There will be
1) Server side pc with touch screen monitor
2) Standard or led display for displaying current number in queue
3) Client side pc’s for controlling queue
The cheapest way is to use led displays. I can create c# app with tcp-ip protocols. But i dunno how to send value to led displays. I can use tablet pc-s with ethernet or wi-fi devices for that purpose (INSTEAD OF LED DISPLAYS). But it is very expensive method. Please help me to choose optimal variant for that purpose.
From a price point of view LED displays are hard to beat, and if you select a decent vendor their reliability and lifespan are close to eternal.
Quite often these LED displays are connected serially, in a multidrop configuration (multiple displays per serial interface, RS485). As to my knowledge that kind of protocol-interface isn’t standarddized, the display vendor will provide a programming manual that explains the commands and data to send to the specific display.
So, you’ll basically be sending strings to a serial port, and that shouldn’t be too hard to do.
It might be a good idea to ask the vendor for a copy of the programmer’s manual and also a test device before deciding on the display you’re going to use. Some display protocols are easier to implement than others (pure RS485 halfduplex protocols are much harder to implement than fullduplex protocols without an external protocol converter or specialized RS485 expansion card in the PC), and/or the range of displays they support (1-line, matrix, ….) and special features (e.g. programmable character sets to create semigraphic special displays), etc.