The cheapest way is to ride with less air in the tires. Down to 90psi should be ok unless you weigh more than the local donut stand.
You could always buy some 700x25s or even wider, although I wouldn't go much beyond 700x28 on 19mm wide rims - just my preference. Wider tires generally allow lower air pressure, however it's mostly dependant on your weight. Too much weight and too little air and you'll be fixing 'snakebite' punctures on a regular basis. If you're down sub 155lbs then even in the 80psi range should be doable - but things may start to get a bit mushy.
If you want to get really comfy on clinchers, the newer generation of wider rims - like the HED C2s run almost like tubulars.
I doubt that anything will roll as well as a nice tubular.