An alternative to Mt Surprise is Undara Lava Tubes.
http://www.undara.com.au/
I wrote to them and this was part of the reply.
"Thank you for your interest in Undara Experience.
We do offer a 15% discount for all group reservations. (To apply for a group reservation you will need to have over 10 people in your group.)
We also have a large open area within our lodge. The ground however is extremely hard, however throughout past experiences it has been a prime area for star gazing and astronomy."
I notice on Google that they have an airstrip which might be suitable for astronomy.(lat -18.191, long 144.611, alt 750m)
http://maps.google.com/maps?f=q&hl=e...=13&iwloc=addr
There prices per adult per night are normally.
Carriage Rooms - Standard* $75
Carriage Rooms - Ensuite* $85
Wilderness Lodge* $25
Swags Tent Village (Excludes linen) $20
Safari Shelter (Includes power) $12
Camp Site or Van Site (no power) $8
* Single room supplements may apply.