You got 20% off. Congratulations. You might have just made the worst deal on your shortlist.

Most buyers I sit with ask about the discount before they ask about the unit. I understand why. A discount reads as better value, and at the supermarket it usually is. Property doesn't work that way. I spent five years selling launches from the developer side, and the discount is part of how a price is presented, not proof that the price is good.

Price and value are two different numbers

In investing there is no single right price. There is the price on the sales offer, and there is the value of the asset you're buying. They don't have to be equal, and most of the money in this market is made or lost in the gap between them.

When the discount is just the increase coming back

Say a developer raises the price of a unit type by 20% over six months, then announces a 10% discount when sales slow.

You're paying less than last month, but more than six months ago. What you got back was part of an increase added on top.

Now take a second developer who never ran a promotion, but whose price from day one was at or below what the asset is worth. Which buyer got the better deal? Not the one with the bigger percentage.

Discounts don't always look like discounts

Waived registration fees, stretched payment plans, free service charges, furniture packages. Some are genuinely useful, but all of them have a cost, and it usually sits inside the price. Convert each one into money and ask: what am I paying in total, and what is it worth?

How I check whether the price is right

Not from the discount percentage. From comparing the unit with the real alternatives at the same budget.

• Actual sold prices in the area. Transactions, not listings.

• Ready units nearby. If a finished unit next door costs the same as an off-plan unit handing over in a few years, the off-plan unit needs a very good reason.

• Other launches at the same budget. Same money, different area, developer or handover date. What else could you own?

• What the unit will earn. Realistic rent after service charges, not the brochure yield.

• Who is building it. A lower price from a developer with a weak delivery record isn't cheaper. You're paying for the risk another way.

It's the same logic I used on waterfront off-plan: a valuable area doesn't make every unit in it a good buy.

The discount isn't part of the property

You can't live in a discount, rent it out or sell it later. It's a relationship between price and value, and if the value isn't there, 30% or 40% off the wrong unit is still the wrong unit.

When you sell, the next buyer won't ask what discount you got. They'll compare your unit with everything else at that price. That's the test your purchase has to pass, so run it before you sign.