It's not just the rods you'll be paying more for, it's also the pistons. Using a non-stock rod length with a non-stock stroke makes the requirement for the pin hole in the piston different, so you have to get custom pistons also. Butler & other Pontiac vendors sell the Eagle crank/rods/Ross piston rotating assemblies as a kit at a good price. If the price of that kit is outside your budget then you really ought to rethink the whole buildup.
You can't just turn a 455 crank down to 3" mains either, because the 3" main blocks have a narrower thrust bearing. In the good old days, people would have the thrust surface on the crank welded up to make it thicker, but welding a cast crank isn't a very good idea in a performance engine as it leads to cracking. Pontiac Dude sells a shim and bearing kit that uses an Olds thrust bearing which is the right width and diameter and it comes with a set of shims that go between the bearing and the block. I don't know anyone who's actually used this kit, but the theory sounds good. You should contact Pontiac Dude about that kit before having the machine work done on your crank though.
Would it be impossible for you to find a 455 short block? That would be your easiest and cheapest route.
If you purchase the Eagle stroker kit, it's about $2000 and that's a balanced rotating assembly. Figure another $800 or so for the machine work which includes cleaning, boring & honing, installing cam bearings, mainline hone if needed, and decking the block. Costs go up from there if you have the shop do any of the assembly for you, and I didn't include costs of head work as you didn't mention your heads or what you're doing in that area.
----------------------------
'72 Formula 400 Lucerne Blue, Blue Deluxe interior - My first car!
'73 Firebird 400/4-speed Black on Black, mix & match.