If all else fails, then I suppose A$18 may have to be spent. 
Hi,
The exchange rate is pretty good at the very moment for us.
It's currently only around $15.25 for each cert . I have just been billed on my card for two certs that I ordered last week.
For the two certs my credit card company have converted 14 GBP to Au $30.51
I have generally paid a lot more than that over all the previous times/years due to the exchange rates, 3 to 1 at it's very worst ( $21) ... So for another money saving tip; keep an eye on the exchange rates each day if you are keen to save a few dollars when ordering anything from Britain. It's good at the moment-The best I've seen it in years.
Regarding cheapest certs, I think you may find that you may have have two choices - price up the local reg, or order from the GRO.
To get a photocopy of a marriage entry (or baptism) from the church registers from the local record office/county archives means you need to know what church so that they can go straight to it- which is cheap just a pound plus in most cases - If you have no idea of the parish/church you will need to pay one of their researchers to look through all the registers in countless churches/parishes in the area which will be a lot more than 7 GBP. If I knew of a cheaper way I would have done it myself -The very cheapest way that I know of would be looking at the church records yourself in the county archives /record office which takes time , which I have done and do when I go back home for holidays
( also refer to andycand's reply to you on record offices as I think he has explained it much better than I have)
Hope that is of some help
Kind regards
