Made a simple LED flower as I wanted to test the conductive thread in use. 
http://instagr.am/p/ITnkq/?ref=nf
http://instagr.am/p/ITn1Z/?ref=nf
This Flower is made using Paeae E-textile Box and some regular cotton + crochet hook. I used LilyPad battery holder, some conductive thread to sew three LED’s on their places and to make a circuit. Also used a battery, three white LED’s, regular cotton thread in two colours and a needle to sew and hook to make the flower. Flower is made of two different pieces so battery holder can fit in between.
What will it cost to do this? I mean without yarn and hooks, just the leds and battery.
battery holder, few LEDs and conductive thread are few euros (perhaps 5 together or so?) and if you need a starting pack then you can star with linked box to try it out and find the most suitable ways for yourself to do things and then get more of those spares.