What a thrilling bargain. A Breville sandwich toaster in the Sainsbury's half-price sale, down from £39.99 to just £19.99. Customers love it. "Got this on special offer – great price and a very good product," says "minker" on the review part of Sainsbury's website (who writes these things?). "Love my new Breville and at half price wish i'd bought 2," gushes "lindy".

Well, "lindy" and "minker" – and all the other Sainsbury's customers who snapped up this "half-price" deal – you might be wondering now if you have been hoodwinked.

The toaster in question is on Breville's own website with a recommended retail price of £19.99. Curry's is selling it for £19.99 while at Pixmania it is £13.60.

As we all know, a manufacturer's RRP is the maximum you ever pay for an item, from which you usually obtain a discount. Sainsbury's, however, appears to have offered it at an extraordinarily high price, then cut it back to the standard price and told customers they were getting a "half-price offer".

But are many of us any longer surprised? The hollow attractions of so many supermarket "deals" are becoming more evident with every week that passes. As a Which? survey found in November, Asda was selling a Goodfella's Deep Pan Pepperoni Pizza at a standard price of about £1, but when it went on to a multibuy deal, the price jumped to £2.50 for one or £4.50 for two. Many multibuys actually left shoppers worse off.

Sainsbury's Breville deal was brought to our attention by a reader, David Godson. I asked the supermarket how it could justify the promotion, and if it felt that shoppers who bought at "half price" might feel a little conned.

Sainsbury's replied: "We aim to be fair and transparent about our pricing," which is the sort of meaningless corporate speak PR people pump out all the time, though it went on to say, in this instance, "we have not been as competitive as our customers would expect".

When Godson originally wrote to Sainsbury's, it told him the toaster had been "on sale in our stores on two separate occasions for a substantial period at £39.99 ... we benchmarked the price of this product against another leading electrical retailers (sic), this is standard industry practice".

It chose to carry on with the promotion. But when Guardian Money told Sainsbury's it would be writing a column on the subject, it changed tack. It says it will now mark the sandwich maker at £19.99 as full market price. That change took place yesterday.

But this is just one, small, example, spotted by a shopper who brought it to our attention. How many other promotions are not quite what they seem? We as consumers simply don't know.

This week, former Labour government minister John Denham, MP, introduced a private members' bill to force the supermarkets to come clean. They would be required to release pricing data, product by product and store by store, enabling shoppers with smartphones to scrutinise special offers, multibuys and "bogofs" while pushing their trolley along the supermarket aisles.

He says: "Supermarkets have a huge advantage over shoppers. While the supermarkets can base their pricing and promotion policies on sophisticated computer analysis of how we shop, most consumers are left shopping around and trying to work out value for money in much the same way they did decades ago.

"My bill will even things up a little, giving consumers more chance of really getting the best deal, without the need for expensive regulation."

As a private members' bill, it stands no chance of making it on the statute book, but is a useful device to draw attention to an issue.

The Office of Fair Trading needs to up its game on this issue, but in the meanwhile please keep sending us not just the daft deals we publish each week, but the misleading ones as well.