Well, it is better than zero to find a block solo mining with less. Otherwise pools wouldn't pay anything for your contribution. The easiest rule of thumb is: How long would it take to earn 25BTC while pool mining? That is approximately how long it would take to find a block solo at neutral luck.
I've always pondered this ideology. If I'm lucky, I CAN generate a block. According to the alternative perspective here:
https://en.bitcoin.it/wiki/Why_pooled_miningthe odds of NOT generating a block month after month are very high, but certainly not zero. Anyway, according to the genesis block calculator, I'd be looking at maybe 7 Bitcoins if I mined through May and June in a pool.