Measure your walls (length and height), drywall comes in 8ft x 4ft sheets, so you will want to plan to use the most full sheets and plan your cuts to waste less. Once you have your measurements, you can add up the cost of drywall sheets and rolls of insulation. Plan some extra expense for 2x4 lumber (in case you open up the walls and find that there are studs that need repairs) and for the drywall screws.
If you're doing the whole house, you'll probably need a large bucket (5gal) of the drywall compound ("mud" as someone else called it). Practice in the back room or somewhere that it won't be so obvious if you mess up, because YES, this is the most time consuming and particular part of the process! Put up compound with a flat, wide trowel, press it into the seams, let it dry ALL THE WAY, then sand. Repeat!
Paint should be the very last thing you even think about! To save costs, look at the "reject" pile at your hardware store (Lowe's or Home Depot)--is there anything there that you like? You can buy a gallon for cheap ($5-$15) and buy another gallon (if needed, for a large room) to match. Mix the two together to make sure the color is consistant. Or, if you're going for an overall neutral, just buy a 5-gallon of some version of beige that you like--this will go a long way! Experts may say that you need 2 gallons for a standard size room, but I've managed to get by with 1 gallon (in a 12x14 room), being very careful of drips and really spreading it out... and not doing a second coat. Sometimes you can find coupons for certain brands of paints (Lowe's does Valspar and Home Depot does Behr, both have worked well for me).
Anyways, there's no way someone can tell you how much it will cost you without knowing your measurements and expectations (how many rooms, different colors, etc). Good luck!
