Markyp, meet
Squeezebox. Squeezebox, meet Markyp. There are a few more advanced wi-fi options available, including Slim Devices' own high-end
Transporter and a very sophisticated offering from
Sonos, but you did say "cost-effective", right?
Each Squeezebox costs around £200 and you need to install the SlimServer software on a resident PC running Windows/OS X/Linux, although if you'd rather not have to keep a PC powered up 24/7, you can also run it off a number of NAS devices (some have SlimServer pre-installed if you're not the Linux hacker type).
Once you have the server up and running, you direct it to your music collection and once scanned and imported into your desired playlists, it will be available to any Squeezebox on your network. You can stream the same tune to multiple 'boxes, or listen to different songs on each one simultaneously. The software also enables remote access, so assuming you have a suitable broadband connection, you can even listen to your home music collection from your office computer.
You can play music in compressed (MP3, MP2, AAC, Ogg Vorbis, WMA, etc...), lossless (Apple Lossless, FLAC, WMA Lossless) or uncompressed (AIFF, WAV, PCM) formats either from your local drives or internet radio. The open-source server software has a pretty active developer community, with a number of
plugins available to extend the abilities of your audio network.