It can be hit and miss.

I look at things this way.
I got through the BA Store and buy 27 Cliff cds for £81. Use a £10 off £50 spend coupon and I'm now getting them for £71 or £2.63 each.

For the 27 cds, I would get 27 x 154 CC Points = 4158 CC Points
At a normal 600 Avios = £2.50 of CC points, that gives me approx 9970 Avios.

Plus 142 Avios through the BA Store for the £71 purchase and anything else if you use a BA Amex.

So that's 10100 Avios for £71.

Get a WT+ sale to New York for £852 and use the 10k Avios to fly CW one-way. The CW fare will be generally £400 minimum more than the WT+ fare so technically for £142, you could be flying CW and saving anything from £250-£1000 (and you'll get 4000-16000 Avios back for the paid flight)

There may even be a small resale value for them to make things cheaper or an Avios offer like the 50% extra one a few months ago.

Some items such as ink may offer 150 points for £8 and after selling them, you only end up with £2 so it's cost you £6 for the 150 points.
The recent GTA5 pre-order thing gave 500 points for £5 = 1200 Avios minimum. £90 o upgrade from WT+ to CW.
My best one was a £1 cd with 500 bonus CC points. Got a few of them and some nice flights out of it.

There are deals out there can can be very good for points/Avios and others where you break even or end up spending just to make a loss. It's pretty much working costs out and seeing if they are worth it to you.
I personally only bought 1 cd and have no intention of trying to buy up to 54 of them as I know I'll be lumbered with them (assuming Tesco would sell that many in one go) and there are better deals out there. But at the end of the day, if you compare BA's price for buying Avios, you're getting them for half of what they charge.