A recent commentor asked what repairs could be made for less than $100. I'm going to assume you've already had an a/c tuneup.
$100 isn't a lot but you can make a dent.
 

Basement - With a few tubes of caulk, some expanding foam and a step stool or ladder start in your basement at any corner looking for cracks or penetrations. Plumbing pipes, or conduit/romex are likely to have gaps. For openings less the a 1/4" caulk will seal well, any larger and the foam will be better. Look for gaps in the framing, between the sill plate and rim joist or where 2 sections of rim meet. Once youv'e made it around the basement and hit all penetrations and cracks start looking for "chases" any area where duct, conduit or venting goes to the attic. If you have a second story and one furnace most likely duct runs from the basement to the attic. An installer will cut the sub floor out large enough to run the duct through, if this gap is left open a tremendous amount of air can end up in the attic. If the gaps are small go ahead and caulk if they are larger do not try and foam it will fall out and end up in your hair or clothes. Trust me it doesn't wash off or out very easily. If the gap is already stuffed with fiberglass pull it out, it is useless for stopping airflow. If the gaps are large you might have to pick up some flat tin stock from the home center. The tin will cut easily with a $10 pair of snips. Cut the tin to shape around the duct leaving enough excess to nail to the subfloor with roofing nails. It doesn't have to be pretty or even one piece. After all you tin is installed caulk every edge of tin.


Living Areas - Working in a finished areas of the house will need a little DIY skill. In a low traffic area of the house pull a section of trim off to see how the window was sealed. Not very long ago it was acceptable to fill the gap between the window and framing with fiberglass. If that's what you have the glass need to be pulled out and replaced with caulk or non-expanding foam if you use the wrong foam it will be a mess at best. If your windows are in need of resealing your doors probably need it as well. This repair will probably be the most noticeable in terms of comfort and drafts, well worth it.
This second tip should not even be attempted unless you are qualified to work with household electricity. All electrical boxes on outside walls should have foam between the box and the sheeting. To check turn off the power to a switch or outlet and pull it out of the box, if it is a plastic box like the one in the photo a small screwdriver through the tabs will let you see if there is foam. If there isn't any foam through the tabs will fill behind the box. Yes you will have to hit every one. More home energy saving can be gained through can light and boot sealing. All in all I can't see this hitting $75.