Suggestion for an alternative to Givi WP401?

Hi , \
I'm in need of an 80 litre bag to hold a tent /sleeping bag etc . The sort of thing you can lash onto a rack with a couple of ROK straps...

The one I fancy seems to be £75, Givi WP410. Nice, however, a tad pricey?
May I ask if anyone can suggest a cheaper alternative?

I bought the Ortleib 109L about 4 years ago and it's been great - no rips or tears etc. Fits decent sized tent and all camping get for two in it. I secure it with 2 ROK straps plus and extra ratchet strap lengthways to compress the width of the bike. The models have changed a little since I bought mine, but this looks similar at £43.50

http://www.cotswoldoutdoor.com/ortlieb-dry-bag-ps490-109l-e1732424?id_colour=

Thanks ..exactly what I was looking for. Appreciated !