I don't think you'll find a stat calculator for mordheim. Standardized costs can be difficult to ascertain due to a number of factors; obviously a high ballistic skill is better in a warband with access to higher quality ranged weapons etc...
You must also take into consideration racial special rules, number of available starting heroes, starting exp of heroes, access to hired swords, weapon selection.. and the list goes on!
So since templated method does not exist, i can only offer some advice based on my experience with Night Goblins. You may wish to seek council from more distinguished warband creators than myself.
I think the best thing to do when creating a warband is to start with something that already exists, and is reasonably accepted as official or close to official. Choose something that fits the stats best, beastmen for Alkemy cat people, or mercenaries for any number of human miniatures.
Once you decide on a model warband, decide on what vision you have for the warband. What makes your warband different enough from the existing ones to even validate this endeavor? For me it was the desire to make a more flavorful and less bland Night Goblin warband, while keeping what worked (namely the snotlings and a couple of skills).
When i started writing my (still incomplete) night goblins, i started with the Karak Azgal warband. I found that the starting costs, stats and exp of the heroes didn't quite mesh with other more official warbands, so i used the forest goblins (nemesis crown) as a base for my hero stats/costs.
Once i had a basic warband i started too introduce some new ideas. I wanted a squig herder hero, and a way to encourage the excessive consumption of madcap mushrooms!
Try to change a few things at a time and play a few games to make sure the departures from the model warband are never too drastic or unbalancing. One thing i may not have done which i would suggest is to include a drawback for each advantage you give.
When it comes to costs, my only advice is to use what already exists, and is it doesn't use gut instincts and play test to see how accurate your gut-magic is!
I tried to stick to what existed but still add a few twists to make it fresh and exciting.