I've created more IT enabled transformation proposals (incl. effort and cost estimates) than I can remember. This comes with the territory of being an architect or consultant for most of my career. Architecture teams are ideal leaders of complex transformation proposal efforts as they have the broad knowledge needed as well as having no incentive to sandbag (or underestimate in case of consultants' proposals). I covered this topic in a LinkedIn event that you can find in the Sample Speeches and Interviews section of my website.
Here are the key messages if the 1 hr video is too much for you
Software, consulting and industry companies I've worked for all use very similar estimating models
The data you need to collect is the same for any of these estimating models (activities, development objects, volumetric/sizing data and high level general factors)
You, not consultants, are best equipped to get the estimating data inputs right
By being able to do estimates yourself you can keep your consultants' proposals in check
The estimate of effort needed is critical. $s needed is then simple math
I've often seen people rely too heavily on consultants to determine what it takes to complete an IT effort. This is an expensive approach. Not to portray consulting companies as villains, but they will do things such as:
Come in low to win the business
Not have enough understanding to properly estimate and thus get it wrong
Base the estimate on what they think you can afford vs what it takes
In all three scenarios above they'll then make it up once they're entrenched in the project by change ordering you to death. This happens not because (well sometimes because) consulting companies are unethical, but rather it happens because it is the only way they can win the business.
Bottom line, make sure you deploy properly skilled and resourced teams for pursuit work or you'll pay it in multiples during and after your implementation.