Barney - as BSD pointed out somewhere much earlier in this thread, Net Present Value is a common approach to valuing mining companies. This basically discounts future cashflow back to todays value - theres an NPV function in excel.
If you have a stab at a conservative resource estimate, work out some per tonne mining costs, remember to do things like factor in recovery rates etc. as well, spread it out over 10 or 15 years depending on how much you think they can pull out a year etc. and you'll get an annual net cashflow to plug into the NPV calcualtion.
Then in that calculation decide on a discount rate (BSD suggest .15 in a previous post on here) and remember to factor in a lead time till mining (??? 3 years??? more?) and up front capital costs. Also when converting your overall company value to a share value, remember to include/dilute for the options that company has on issue. e.g. CDU has about 50 million shares but also about 50 million options that are well in the money so effectivley form part of the companies issued capital. (and maybe another 10 million if the shareholders meeting approves the issue to the directors).
You'll find in tools like the comsec company research page show the market caps based on only issued listed shares, and not the options so do your own calcs on market cap for companies based on the issued capital information described in the companies recent appendix 3b announcements.
There's a feasability study somewhere amongst copper co's (CUO) recent ASX reports - look through that and you'll see the long term copper price they've used for the base case for the CUO feasability study, and also things like cost per lb estimates , capital costs for startup etc. Remember to factor in exchange rates etc. as well.
What you'll find is that you end up with about 10 numbers you can tweak (including ore tonnage and grade) that will give you wildly varying values anywhere from $1 to $50.
Other ways to get some idea of potential value is to compare the ore size and grade to similar companies - copperco isn't a bad one to compare to I reckon - its a pure copper company, still a year away from production but also at least a year further along the path to production than CDU, has relatively similar sorts of grades to what CDU are talking about etc. etc. but they have a feasability study complete, various other agreements in place including financing, a proper resource estimate etc. Remember when comparing two companies you should be comparing they're fully diluted market capitalisation (i.e. not share price - because different companies have different amounts of issued capital so share price isn't a reflection of relative value unless you factor in the amount of shares each co has - i.e. market cap).
Look at how the market values them and to a pro-rata comparison to CDU based on what you think CDU's ore and grade will be - however remember CDU is still early days - they don't have anything more than an inferred JORC at the moment - no metallurgy, no finance, no indicated/measured reserves, no feasability study, no finance etc.
The big question mark is how much copper has CDU actually found - both in terms of tonnes of ore as well as the overall grade across the entire find.
As others have said, investing in companies at this stage is risky.
With any stocks you invest in, beware of ramping and price spikes - make your own decisions on a value - then be conservative - because even if its theoretically worth more the market might still not end up valuing them that way.
Remember as well that copper prices rise and fall, interest rates rise and fall, mine startup capital costs blow out as well as timeframes, etc. etc. so nothing is certain.
One piece of advice - if a stock has risen very strongly for a few days and volumes are high, its probably a risky time for a novice to enter it. Another thing to get a handle on is stoplosses. If you do decide to enter on a punt you need to set a stop loss to prevent the situation you had where you hold and hold until eventually panicking and selling at the bottom.
You'll find that emotions are the biggest factor to control in investing/trading - so having your own ideas on value, price, and - prior to entering a trade - knowing the conditions under which you'll buy and sell (both positive and negative) and then sticking to them are important.