I thought one of the Realtors might have chided in, but since they haven't yet, I will respond to the best of my knowledge. I bought a home for $300K in Boise's unincorporated area and my taxes were approx. $2500 a yr with the homeowner exemption several yrs ago at $75K. Taxes are based on the current year's assessed value when you first purchase a home, less the current homeowners' exemption. The 2nd year is normally based on the purchase price you paid, less the current homeowners' exemption (which I haven't looked up for this year, but someone will probably chide in with that number soon). Every year thereafter is based on current market conditions less the homeowners' exemption.
When you purchase a home, you must fill out the homeowners' exemption form to pay less on property taxes. In Ada county, the easiest way to figure out what your taxes will be is too look
at the address of the home you want to purchase on the assessor website and see what the current assessed value is for this year or last, add the exemption amount and compare that number to the sales price. In your case $300K. The Ada county assessor's website really should give you a pretty good idea how much your taxes will be.
When I was doing research, Ada county had lower property taxes than Canyon county. Unincorporated Boise was cheaper than incorporated Boise (in town), and Eagle at that time (mid 2000's) was the lowest of Ada county. All in all, I found property taxes to be very fair and reasonable. HOA's weren't too bad either, though many folks are opposed to HOA's.
Hope that helps Binkus! I do a lot of research and analysis, so sometimes it is easier for me to figure these things out because I've done it so much.