For the amount you mention, you could have bought/made the whole "computerized" darkroom of the type I have.
It takes an older Palm Pilot with serial cradle (i.e. Palm V or older, if you're willing to use older battery-powered Palm), an electric relay (which does the actual switching on/off), and a cheap guitar pedal (I paid about $14 for mine, new).
Illustrations and description can be found on my Web site,
HERE.
The only downside I can think of is that the device can time only full seconds - i.e. you can't program a time of 4.5 seconds - it's either 4 or 5 seconds - but I don't find that to be a problem.
In short, this solution can be hooked up to ANY enlarger, and the wonderful thing with Palm Pilot combined with Fototimer software is that you can "program" a full sequence of e.g. test strips - i.e. 4 exposures of 2-3 seconds, plus a longer final one. Something like 2s. - 2s. - 2s. - 2s. - 14s. for exposure of 5 test strips, then 10 sec. pause (to remove the strip from the easel), then 2 min. in the developer, then 30 sec. in the stop bath, then about 2 min for the fixing - all chained up and automated, with beeps to warn you of the end of each step, and adjustable pauses in between...
And it also does what you're asking: one press on the foot switch starts the process, and if you want to stop it before it's run its full programmed time, just press the foot switch again.
It's worth considering if you really require such functionality. Even if you're not handy with soldering (I'm not really good with that stuff), it's not really complicated to do. My total cost was something like $30-$40 (not including the Palm Pilot, which I already had).
Regards,
Denis