If you do nothing to control the temp it will easily get 10* warmer. Most ale yeasts are at their best between 64-70* or so.
I had been just monitoring the water temp with a floating thermometer and keeping the water around 64*. When I bought a fermometer it stated that the temp inside was still about 4* warmer.
A few degrees can mean the difference between an OK beer and a good beer, or at the higher end, whether or not you have created any fusel alcohols, which are nasty!
I live in central Texas where it is quite hot, and I can't keep my house that low or else SWMBO gets very mad. It's roughly 75* in mine. But I have an extra bedroom that seems to be a little cooler and so I keep all of the beer stuff, camping, hunting, etc. in there.
Look into the swamp cooler. Your house temp won't mean much. Unless you make a Belgian beer where the temps can get into the 80's.
I, too, am a budget brewer, and do what I can to reduce my costs or find things to repurpose, such as using 2 qt juice containers for yeast starters and harvesting the yeast after I bottle a batch. I also reuse 1 gal vinegar jugs for chilling top off water, wort chilling water, and harvesting yeast.
Look into "washing yeast" to find how to reuse your yeast. Big cost savings right there. Or you can do as I am going to try next, which is to just make a larger yeast starter and save a 1/3 of it so as not to have to go through the hassle of "washing" the yeast. It only costs about $0.50 in DME to make enough extra....
Here's the floating thermometer I use for $5.95:
Do you know if your LHBS sells grains in exacting amounts?
You ought to compare prices with MoreBeer
. Compared to the few others I originally looked into the prices were similar, but they gave free shipping on orders of $59+, which tipped it favorably. But some prices on things were a little better with MoreBeer. The only downside was selling grains by the pound, and LME by the pound starting with 4 lb bags, and a lack of wheat LME.
Any idea as to what type of yeast you'll be buying?
What kind of beer are you making?