Ready mashed potato ripoff

I am on Weightwatchers (lost 2 stone and half a pound since July) so have been getting frugal with meals. One thing I have been avoiding is mashed potato - love the stuff dripping in butter (I know how I got this big! lol) and theres been grumblings from youngest dd and oh as they are sick of new pots and want mash. Had a look in Asda and a small pack of fresh mash was £1 and it would have barely fed those 2.....so I bought a bag of potatos for £1 from Asda and mashed them myself and froze them in 2 big portion sizes - so I got 5 out of the bought potato which even considering the gas it took to cook them is still a much better deal. However, I love swede and to make mine and my eldests mash I use two of the smaller ones at 85p each. I had a look at the ready prepared turnip and got 2 bags of it for £1 and there was enough for 3 of us and another 3 person portion is now in the freezer! So although it might seem cheaper always to prepare your own veg always keep an eye out for ready prepared stuff that works out cheaper.....