ArcGIS is worth considering because it can also be handed over to the client to map their hardscapes, above and below ground utilities, irrigation, lighting and everything else so that they'll eventually have all their systems and landscaping captured and inventoried in one GIS collection program.
As to giving discounts on recommended work, personally I feel obliged to adhere strictly to objective consulting until completion of that assignment, to avoid the appearance of having my thumb on the scales in making recommendations for work. Once the consulting assignment is complete, I'm willing then to submit a blind proposal which matches the specifications which have been produced through the inventory and management plan. I figure if my pricing is relatively in line with other proposals they receive, they may see an advantage in having the chef who helped them create a cookbook, actually prepare the recipes. Offering a discount could be seen as seeking an unfair advantage by discouraging them from due diligence in requesting legitimate bids on the work. Best to avoid slippery slopes and establish yourself as an ethical consultant who can always be relied to stay on the high road.
My perspective is that since I only have one head, I'm only willing to wear one hat at a time. My contractors hat never goes on until after my consultants hat has come off due to completion of the consulting assignment.
Pricing is going to be based upon how long it's going to take you to do the field work for the inventory, and then to compile the yrequisite reports. Inventory pricing is like a menu in a Chinese restaurant. If they just want species, size and location, that's one price. If they want condition ratings, that's more. If they want recommendations for retention...more. Recommendations for treatments to improve condition...more. Tags..more.
Ultimately, your client is going to decide how much data they want to mine. You can always offer the "Happy Family" discount if they select multiple data sets to be collected in the field and compiled in your report.