I have been mapping and using photographs (in GIS software) for over 10 years to estimate acreage for environmental work. Phone apps, online aerial photos, and GIS software are all useful. The cost of using a GPS unit in the field goes up with the accuracy of the unit. I use a unit capable of sub-meter accuracy, which costs about $2000.00 used, and the amount of error can run as much as 20%, per the experience of a surveyor I did some work for. GPS units can also be inaccurate when you have tree cover around the field edges.
I have also been using some Trimble Ag mapping products that you can put in your tractor, truck, etc. that are useful too. I believe the last one the company bought was about $1100.00.
So, keep in mind that most tools that are economical are less accurate. They should be considered an estimate and not an absolute.