That is great price for nice mould...... perhaps you could offer ALU mould making service for the forum

I can assure you Steve, if you can get propped aluminium for this tool for less than it would take to make a unimould tool, it has to be from an illegal source. And cnc tooling is only cheap if you "know someone". Even if you do it yourself it get's more expensive than you might think.

Depends on the geometry. If it is a simple straight octagonal tube, then I'd just get some octagonal aluminium extrusion, split it with temporary barriers on opposing edges, and make the mould as per Warren's linked tutorial.

If the pattern was less simple, and for some reason octagonal extrusion wasn't the right shape or the tube needs a bend in it, then I would build the pattern by hand in the traditional way with foam and guide templates. I do the same thing making custom intercooler piping for very complex pipes, and there's generally about a days work in getting the form right. An octagonal tube would be even easier than a round tube for this, as you don't need to try to sand an even round surface using flat tools.

I'm yet to discover something that can't be made this way. Don't get me wrong, CNC's are wonderful and can make your life a hell of a lot easier, but I'm also lucky enough to own one. If you don't have access to a CNC, and you aren't looking for volume production or engineering tolerance accuracy, then making stuff by hand is perfectly suitable.

You guys must get aluminium a lot cheaper than we do in Aus. I've got prices on a few one-off machined moulds, and something this size you would be looking at AU$2000 minimum - IF the machine shops want to do it at all for less than 5 parts.

Crazy to think composite tooling would cost anywhere near that much, you'd be closer to $200 in materials. More of an investment in your own time sure, but that's generally the case with one-offs. A 2-piece split mould for this would be dead easy to make without any CNC involvement at all. 

It all depends on what you mean by 'outside surface finish is important' - MDF can get you shiny provided you want to do a little painting after.  MDF will not have nearly the dimensional perfection of aluminum either - but it will totally work if you seal it well with epoxy and use a good release system over primer - or a surface film (like PTFE film) for release.

If you're trying to do this cheap and tolerance and strength aren't super critical, you could probably do fine with a bagged wet-layup in MDF molds - things would be tight doing the layup though!  You could even make the molds by hand with a table-saw and some body filler.  It all depends on how good you need it. 

Infusion would be an option but 500mm is a bit far to infuse and it's more trouble.  Infusing across the tube would be fine if you design-in a thicker split-line for the two outermost plies of carbon to turn out and act as resin flow media and vacuum outlet.  This would be easier and less risky than end-to-end but there is a small cosmetic penalty.  Everybody is suggesting pre-preg because for something this small it is much easier to control and keep tidy.

 What type of laminate are you after?  Would a 60% uni in the 0-direction and the rest +/-45 and one woven surface ply work? 

Like: (from the outside)
200g at 0/90
300g uni at 0
300g uni at 0
200g at +/-45
300g uni at 0
200g at +/-45
300g uni at 0
200g at +/-45

This would be easy enough to wet-lay with the +/- 45's lapped - or laid out in one long staggered stack the full circumference of the tube and rolled around the internal bag.  It might be messy with wet-layup but you could also use braided sleeve for the +/- plies.  

Two pieces is tough - too bad you can't buy it somewhere.  Hexagonal would be an off the shelf thing but I've never seen octagonal!

The problem is that once you go down the short-cut road you'll never get anything as nice as pre-preg in Steve's aluminum mold - but it'll take more time and probably cost almost the same money in the end.  Wood CNCs can cut aluminum if you do it right - and machine shops could whip this out pretty fast.  And low temp pre-preg can be cured in aluminum molds with one of those silicone heater strips like used for 3D printer beds stuck to one mold half.

I need only two.
I would like to avoid prepreg as i dont have long enough oven.
But i have access to wood grade cnc mil, how about mdf mould?

But how would you make a 'traditional split mould', out of fibreglass? If so, how do you make the pattern? Starts to get expensive and approaching the cost of just making an aluminium mould, IMO :-)

As the OP required a nice external finish aluminium is also a good candidate as it polishes easily to a mirror finish without the need for gel coats etc. Once you start to start to add up all the processes, time and materials required to make a traditional pattern and mould, the CNC aluminum mould (no pattern required) gets better and better IMO :-) If the OP requires multiple copies of his item them an aluminium mould is a no brainer as far as I am concerned.