I believe landscape fabric is 100% worth the cost! I have two big flower and bush beds in my front yard. The bed with fabric is weed free, the second bed with no fabric is all weeds. Both beds have mulch.
Landscape fabric has simplified my life. Without it I'd have weeds galore. My property looks wonderful because of this miracle product! My shrubs are happier without unwanted growth around them. I can spend my time doing more pleasant activities because I don't have to do property maintenance constantly.
Landscape fabric is not the most effective means of keeping weeds out of the lawn or garden because each year new seedlings will take root in the soil that sits on top of the fabric. A much better way of controlling weeds is the yearly application of wood mulch.
I use landscape fabric to prevent weeds and to keep my plants moist, but another added benefit is your top soil will not wash away in a rain storm. It is very much worth installing.
Since I re-landscaped and put down the fabric, the only weeds I have had to pull are around the edges, and I have only had to pull a few there. It is well worth it!