Send and receive data from arduino clients to a webserver over REST

I am a total newbie on Arduino but before I buy one I would very much appreciate some suggestions on what to go for.

In my project I would like to have several Arduino clients that all are connected to the network over wifi.
The clients should all be able to send and reveice data to/from a web server. In a best case scenario they should all
be able to send and receive data over REST calls.

I had a look on the REST API for Arduino and the CC3000 WiFi Chip. This seems to be a priceworthy solution but after looking in to the examples it looks like the Arduino chip can either act as a REST server OR as client? In that case as I understand it the Arduino cant both recieve and send data with this api? Is that correct?

It has to be a rather cheap solution since I need many Arduino clients and the wifi shield for Arduino is a bit too expensive.
Perhaps are there other solutions that I am no aware of?

Thanks in advance!

REST API

You have any links to information on this? An arduino with a w5100 ethernet shield can run combined http server/client code.

this is the rest api I was talking about: