Luckily I already have one and my spare is missing in action on the Isle of Man, submerged in a Peat Bog known as the Black Dub.
when I loaned mine out to a Irish lad for the Manx GP I found myself entering a race with New Era and having to rent one, £10 for the hire and £50 deposit. Mounting Brackets were £5 to buy but mine was already fitted.
So on the morning of race day I handed over £60 and my ACU licence and at the end of the day I returned the transponder and got £50 and my licence back.
May be worth a call to New ERA to see what tis years Policy and pricing is.
Here is the North Glos info on it including suppliers
http://www.ngroadracing.org/Pdf/transponders_nov04.pdf