For invoicing I would use quick books, the easiest program around. As to how much to charge, what is your time worth? The overall value of any construction project is public knowledge, the price the contractor is being paid to build the project is stated on the building permit - that information is available at the county or city building and safety department.
Lets say your fee is based on that value and is 1/4 of 1% of the building permit value. A $2,000,000 project would have a fee of $5,000. Based on my construction experience, that project would take about 6 months to complete - so your fee would be about $200/week.
If I were estimating my return on that fee, I would figure about 1 - 1.5 hours to travel to the site, document the progress and return to the office or another site. That weekly time plus 8 hours to organize and print the final report would equal a total 47 hours if the visit was once a week or 86 hours if the visit was twice a week.
Your return would either be $106/hour or $58/hour depending on the frequency of the visits. You alone must decide if that is enough for your time, travel and photography expenses.
When approaching a prospective construction customer, I would quote the fee in price per visit or week rather than a total for the job, keeps the number lower and reduces the sticker shock.
Like I said initially, one small project, either free or performed for a small fee (to cover gas, etc.) will tell you three important things. One, what is your cost/time commitment on the project and two, is this a viable market and three is this worth the work?
