You could buy a second-hand box on Ebay quite cheap but you need a Dual LNB on the dish so that it can be connected to both Digiboxes.
There is another way, the really cheap way.
It is perfectly possible to connect two digiboxes to a single LNB dish setup provided you use good quality cable and good quality connectors to split the signal.
You can then have two different channels at the same time.
The drawback is that you can only have two different channels if they are both on the same polarity.
With analogue Vertical polarity has priority. I.e. if you were watching a Horizontal channel and someone turns the other receiver onto a Vertical channel, your receiver will go vertical too and you loose your channel. Similarly if you are watching a Vertical channel and someone else changes the other receiver to a Horizontal channel, they will not be able to get the horizontal channel until you turn your receiver off or change to a vertical channel.
With SkyDigital this same Vertical priority situation exists, if your receiver goes Vertical because of the other receiver then the 'No Satellite receiver is being received' caption will be put up. And if one receiver is changes to a Horizontal channel whilst the other is on a vertical channel then the 'Technical Fault' caption will be displayed.
Whether or not this situation will work for you depends on the particular combination of channels and services you want to watch.
In my house, because of the particular channels that myself and my parents like to watch, this setup works fine and we hardly ever have clashes. In your case, it may work same as it does for me then again it may not.
Perhaps it's an idea to pop along to the Astra website and check out which channels have which polarity and then look at what the viewing habits of the people using each receiver are likely to be.
If you do need to go for a multifeed LNB, then you will need once designed for your minidish, your standard clamped on LNB won't fit a Sky dish. I think the only multi output LNB that you can fit to a Sky minidish is a 4-output one. This is the Sky+ LNB which provides two outputs for the Sky+ box whilst still allowing you to run your existing Sky box elsewhere. The 4th output is there in case you ever get a second Sky+ box. Of course you can also use it to run 2, 3, or 4 normal Sky boxes too.
I don't know about pricing but if it's pricey it may be cheaper to get a conventional dish (which will give you better signal strength aswell by being larger) and a convential dual feed Universal LNB.