Jonathon,
In addition to Gools post above, I cannot give you a quantifiable percentage figure

to add as a premium over and above a site locked by dwellings on either side.
I also like corners for the flexibility and street appeal.
You really need to ascertain your walk away bid (purchase price) based upon your feasibility.
Whether you are selling one or both, or keeping for the portfolio, you need an idea of gross realisation of end product on todays sales data.....do not try to second guess the future (corner based prices if possible) and IMHO allow 25-30 % return. Banks will not accept the traditional 18-20 % return in this credit climate.
To this end irrespective of whether you keep for yourself as rental stock or wish to onsell, you still must not over-pay (regardless of whether it is a corner or not). Your numbers are crucial.
Also depending upon your desired exit strategy with the development, you need to check with your accountant regarding the margin scheme for GST and if (at auction) the vendor is willing to apply the margin scheme to the contract should you so desire and are the successful purchaser.
Also, bearing in mind your anticipated purchase price and intended future development plans, have you got any reasonable idea of your likely finance position from a bank or broker?
Haven't really answered your question in any quantifiable manner and actually asked more of you, however hopefully this assists with your investigations and diligence moving forward.
