How to cut expenses on cat litter? Help PLZ?

I have 3 cats a 12&1/2 old adopted male tiger cat a

3 year old female rescue Black & White Kitty & We just got a surrender

a few month back (from craigslist) cat in February 8 2011 she is female maybe 6-8 years old they are all fixed all indoor cats all free fed wet and dry food .They are all FeLV and FIV negative, dewormed ,flee treated.

My son and I are a low income family but we do what we can when we can .Tigga the oldest has been with us since 1998 and the 3 year old 2008.They eat alot but i came up with a cost effective solution to that. But the litter no lie i am going threw 3 Bags of litter a week at $5.00 a bag for the Biggest bag

that's $15.00 a week $60.00 a month.I need a cheaper solution. I do not use scoop able litter or scented Minny sneezes .I use TIDY CATS Non-Clumping Instant Action its better for there eyes its 99% dust free minny is really bad with dust in the eyes and The two oldest have some

respiratory issues tigga mainly he is asthmatic and i scoop(clean, poop a scoop) up to 3 times daily.Any solutions ?Please no rude comments like get rid of them or if you couldn't afford them why get them.I am affording them its just we are on a fixed income .I am a 39 year old single widowed mom trying to send a 18 year old to collegee . when 18 years ago the medical community said he would never read or write because he was diagnosed with multiple different abilities mainly aspergers syndrome. So so we live on a budget is all so please no rude comments please.Positive helpful advice .thank you gsmom