Thanks for pointing me toward thiat fuzzy logic unit. I used its specs to decide what I really need for my tiny garden. I decided that the essential requirements are that

1) it can sense ppm of co2 and control the release of gas to maintain that level;

2) It runs only during daylight hours

3) During daylight hours, I can choose blocks of time during which it will operate.

I found two items on ebay that combined with simple 24-hr timers should do all that for under $200. One item is a two-stage tank valve with a flow meter and an electric solenoid. Cost (shipped) $93.20. The other is a co2 monitor panel that is adjustable for the ppm range needed. It includes a relay which will trip off when the level I want is reached. Cost (shipped) $98.00. I connect the two pieces, plug into a 24-hour timer and plug that into my light timer. By synching the two timers I can choose how long and during what part of the day I want to enrich the atmosphere.

You'll immediately notice that I'm not looking at venting for heat or humidity, which seems to be a basic component for most of these greenhouse regulators. I'm at the mid-day of my veg phase right now, with 69f and 39%rh. In the middel of summer, I've only recorded 90f back there once. Heat isn't a problem in Alaska . . .