what are the cheapest ways to boost a stat (e.g. wisdom) in early levels (~5th level)?
Food
If I am your DM, I will accept cash. But free food works as well.
Oh, wait, you wanted something IN game. Oh, okay... well... Go bother a banshee.
She'll scream at you, you'll age a few catagories, you'll gain wisdom. You'll become a crotchy old fart, sit on the front porch of your church, and wave your cane at the younger adventures while you yell at them to get off your front dungeon.
They are trampling your Azaleas, after all.
You could go up in level, and get a +1 every 4 levels to a stat of your choice. That would also be cheap.
You could take a template, that would cost you a Level adjustment, but cheap as far as GP is concerned.
You could become a crack addict and start taking BACCARAN for the 1d4+1 wisdom bonus for 1d10+15 minutes and it only costs 10gp.
Find a way to use the DMG 3.0 wisdom potion 1d4+1 wisdom for 3 hours for 300 gp.
A wonderous item will give you a +2 to X stat for 4,000 gp. Ask to make it Your class and alignment only -30%, then charged and only 3 times a day. Who gets into more then three fights a day? You'll have a +4 to wisdom for 3 minutes/2 times a day and it'll only be 1680 retail.
Techincally,if you are at an odd number, you only need to make your wisdom an even number to get an additional bonus. +1 wisdom all the time, 1000gp. Of, make the previous suggestion a +1 instead of a +2. Only 420 gp for a +1 wisdom 3 minutes/3 times a day.
You could buy a suit of OWLFEATHER ARMOR. Then buy off everything BUT the 1/day +4 bonus to wisdom for 9 minutes and and add a 30% reduction because it will only work for your class and alignment. Should get it down to about 1140 gp. (seriously. Mathmatically, that is one screwed up suit of armor) Use the MIC version.