I am about to put playsand in my tank... the main thing to make sure of is that it is non-toxic. plenty of people have used it on this forum..
Not sure about the stuff from diy places, i have heard about some of the builders sand not being quite right, but I would wait for confirmation from others.
Where are you based.. in the UK you can buy 15kg bags from argos for £2.49
Cheers
Squid