Re: K3 front end
 

It depends if you buy a complete front end, or if you can source the parts separately as I did. Mine didn’t work out that expensive to be fair, a little while ago but probably came in less than £500.
I managed to get the forks, yokes and wheel in one lot from a gsxr1000k2. can’t remember how much now but I know it was less than £250. I then got callipers from a pre radial mount gsxr 750 (600’s are the same) for about £100. I then had to get tl1000 clip ons as the gsxr ones foul the tank, but you won’t need those for a straight bar conversion. And a mudguard but I bought an expensive Harris carbon fibre one and could have got a plastic one for significantly less.

You will need to cut the speedo wires and splice them to a sensor that will need mounting inside the mudguard and then you will need to bond some magnets in the rings on the floating front disc to make the speedo work.

Or you can get the internals on the original forks uprated as bibio says, but the chrome on the fork tubes started pitting and rusting pretty quickly on mine, I always had knocking through the headstock from new, and the standard brakes are not really up to the job in my opinion which is why I decided to go the gixxer front end swap.