Lol! I think u'll get more than an 8th off any plant grown under 125W, tho to be fair we ain't talking about sick poundage either!! And sadly the boxes have lied to us - they aren't anywhere near a 400W HPS! (Bastards!!)
Obviously guessing what amount a particular light will give u is totally random & dependent on so many other grow factors, but since u probably just want a figure, i'd estimate 125W would give enough light coverage to grow a 2ft (ish) plant, and u'd probably have to stake & tie it abit so to allow all parts of the plant to receive light. (Light from a 125W CFL won't penetrate far past whatever it first hits) Assuming u keep it healthy & strong u'd get roughly 15g to 25g of dry bud. U could probably fit 2 plants under the CFL but then they're likely not gonna be able to get so close to it & will probably stretch & in turn will each give less of a return, maybe 10g to 15g each. (oh, and don't read that as a 125W CFL won't grow plants over 2foot - u could grow a 10foot plant with a CFL but it'd be majorly stretchy, weak & totally pointless!)
U will get weed back off that light but it won't be fat, dense nuggets like u get with HPS. I'd say give it a bash, see what happens & u'll certainly learn alot about the plant along the way anyway. And its this experience which is arguably more important than having good grow equipment straight away.
(And obviously i could be totally wrong with my numbers - but thats the trouble with guessing figures based on minimal input. See what numbers anyone else may quote, but the only real way to find out is to just do it!)