What category of pipelines? Water, or oil & gas, etc.?
General size and cost range of your projects will limit the degree of detail you can aspire to achieve, as will your experience in similar, recent projects, constructed in the same region.
For a 1000 ft long project you might reach +/- 10% after only building two or 3 of them. For a 1500km hot oil line with 6 pump stations and a marine terminal, you may only know the cost within +/- 10% 6 months after completion, or maybe never.
Type of pipe, PE, steel, cement lined concrete?
Is amount of pipe large enough for a direct purchase mill order, or will you buy from a 3rd party supplier? Determine $/ton on that basis.
Determine pipe wall based on design code and pressure, tons/ft. Multiply.
Many specific items are estimated using factors applied to pipe cost.
Such as adding 5% for pipe damages, drop offs etc. 5% for valves & fittings, etc.
Adding 4 or 5% to account for true topography, rather than flat-earth distances.
Estimates depend a lot on what you know and still don't know about the design when you make the estimate. Is the exact route approved, surveyed and land purchased or leased, or is there still potential for significant rerouting and need for pipe reserves to go around Indian reservations, etc, rather than straight through. Do you have crossing permits in hand now.
Estimates can depend on current state of the design.
Are transportation distances between pipe mill(s) and site established.
How many work camps will you need.
Where will pipe be temporarily stored.
Where is the coating yard.
Manual or automatic welding.
Add costs for pump or compressor stations. Typically determined by a $/hp number.
Adders for numbers and types of crossings. Farm roads, state highways, federal highway crossings. Stream, river, shallow, wide, or deep? Lake crossings.
Adding for HDD crossings.
Basic add for construction is to double the pipe cost, then multiply by regional factors.
Construction costs can vary widely between regions, say for 16-24", from 500,000/mile in the western US to 5,000,000/mile, or more, in the Northeast US. You must get a handle on construction costs in the region of construction and multiply basic cost above by that regional factor.
There are terrain factors for prairie, hills, swamp, mountain and city and town works.
Terminals can be estimated mostly based on number of tanks and tank volume category.
The Oil and Gas journal publishes pipeline costs yearly.
Additional gas pipeline cost data for specific projects can be found in FERC archives.
When you master the above method, it should get you to a level 1 estimate of +/- 50%
Anything more accurate involves getting budget estimates, price estimates, or firm purchase agreements, in that order as the project progresses.
Pipeline estimating isn't suitable as a part time activity. It is a dedicated engineering service.
It takes a lot of work to keep costs current.
Even similar projects are different.
There's a lot of art to it and learning a bit of black magic and tylanol also help.