Engine balancing question

Depends on what balance option you go with.
Zero
28oz imbalance

Zero or neutral balance will cost more due to the extra steps and material required, and how your pistons/rods/rings/bearings affect overall bobweight.

28oz imbalance should on average 180.00. It's best to take your stuff to a very reputable shop, one in which you've had a chance to talk with a few of their customers.

Going 0 or Internal is not worth it on a stock rotating assembly, it will cost a fortune in Mallory/heavy metal to make that happen.

Your stock motor is 50oz, your balancer and flywheel have to match which they do stock. Since this appears to be a budget 306 just take the new pistons, wrist pins and rings along with 1 set of rod bearings to the machine shop, along with your crank, and 1 old rod/piston. Stock pins are pressed on to the pistons so you will need to have the old pins pressed out and then have the new pistons installed on the old rods. I would recondition the rods while they are out and add ARp rod bolts....any good machine shop can do this with no issues. Expect to pay around $150-175 for balancing, plus labor to press off the old pistons and install the new ones, have the block tanked, bored/honed, cam bearings and have the mains checked to see if you need an align hone. All said and done your looking at $600 plus.

If it's within your budget, consider balancing to 28oz. My reasoning... putting a budget on a build = less power and taking weight off the rotating assembly increases engine acceleration and that is a good thing to happen.
You don't have to have the most power to be quick, you just have to accelerate faster than the other guy.