Capitalist,
I have just bought an IP in Perth and went through the same issue. I did the following in order to get a feel for land value.
Got a list from the Val Gen office of past sales for the suburb (close to city, very established 1920's+) for the last 5 years ($40.00) - VGO data has land area, no bedrooms etc. etc.
Drove to each of the prior sales and had a look from the outside.
Looked at everything else for sale in the suburb and determined $/m2 (like you, no vacant land to compare, but it's a good start and if you see enough you can normally get a feel easily for what's a good house, what's a new reno, what needs a tart up versus what's a pull down).
Looked at as much as I could for sale in the suburbs surrounding, got the VGO data and again determined $/m2 - in surrounding suburbs I was able to find spare land, or land that was possible to be subdivided - I found subdivisions or potential subdivisions a great way to get a feel for land values
Note that within reason, smaller parcels of land will most often have a higher $/m2 value
I found a variance in my suburb of 665 $/m2 to 1,623 $/m2, but that's where the looking around comes in.
The above got me comfortable with paying a range toward 1,000/m2 based on certain criteria, the right 'part' of the suburb, the right type of street etc. From there and the anecdotal information I picked up from the rest of the work, I was able to judge (I hope) reasonable value.
I further had a good look at the houses that had sold in the same street and found out that a house across the street from mine sold for 1,023/m2 in April this year, but was a 670m2 block. The house I bought worked out at 820/m2, but was a 986 block with some work to do. Spoke to the owner of the 670, found out a bit about their house condition at purchase and was able to compare from there. I looked at the work I expected to do (grossly underinflated the cost no doubt!) and could then get to a comparable $/m2 on a similar condition.
In the above I also spoke a number of RE's to get their views on land values etc. I found land value was the best starting point for me.
I did this in 5 days which included a weekend as I was only visiting and had very little time.
I found it amazing how helpful people were able to be if you asked the right questions.
Hope that helps.
Cheers,
Ralph