DIY...
I looked exactly what you looked into....
I found Concrete was more expensive, almost 1 cubic meter (about $300 from concrete taxi) and about $60 for a formatube..
I got my steel from direct steel at ipswich and total cost was $79
it consisted of
300 x 300 x 20mm mild steel base plate
2of 200 x 200 x 20mm mild steel top plates
165mm OD pipe ( i actually went 150 od pipe as they were out of stock of the 165)
+ bolts total cost is about $100... even if you factor in a cheap bunnings welder + gear at about + $150 or so its still cheaper then concrete and a commercial product.
Stability should be about equal as well, especially if filled with sand. if its out in the middle of the yard on a windy night things might be dodgy. there are members on here who use smaller diameter steel and its still uber stable.
linked is a cad drawing of it.
if you cant DIY find someone who can weld for u.. clean up only needs a cheap angle grinder. IIS members are uber helpful. If you willing to wait a few months i will make mine, i will make you 1 as well.
Pier
http://steve000.dyndns.org/images/200mmpier.pdf
Pier with my scope on it. scope and mount are verrrrrry close to scale
http://steve000.dyndns.org/images/200mmpierscope2.pdf