.... It's relative to several factors.
#1 Ground Conditions. Highly mineralized ground will limit the depth reached to possible targets. Here in SW Florida, our sandy soil has hardly any mineralization. An MXT with the standard 9.5 concentric coil, gain at +3, will reach out and touch a Merc dime at 10 to 12 inches. In other parts of the country you'd be lucky to read the same merc dime at 4 to 6 inches.
#2 Type of detector, VLF (very low frequency) or PI (Pulse Induction). PI's will seek targets deeper but offer little to no discrimination. VLF machines allow the user to discriminate between ferrous and non ferrous targets and even varying levels of non ferrous. The higher the level of discrimination used the less depth you will receive ... it's not a huge difference, but there is a difference.
#3 Coil size and type. To a certain point, the larger the coil, the deeper it will seek targets. Too large a coil makes it difficult to pin point smaller objects as the coil may react to several targets beneath the coil .... oval shaped and / or DD coils seem to offer a bit better depth.
Unless the soil has been plowed or landscaped / filled, most "dropped / lost" older targets will be within 4 to 8 inches as to max depth even in woods where leaves have fallen and rotted for hundreds of years. In plowed fields in the UK, I've found Roman and Medieval coins on the surface to as much as 10 to 12 inches deep.
Make sure you select a machine that offers both manual and automatic ground balancing, easy and quick to use controls, good light weight and ergonomics so it's fun to use ALL DAY LONG! Buy the best machine for the type of hunting you plan to do the most, that current cash flow allows.
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