These are my own recommendations of great watches that are available for a under $100 to start your watch collection. While you cannot be too specific such as the type of movement as well as type of crystal there is still a lot of good options under this price if you are just entering the world of watch collecting. All the prices are in USD.
Casio G-Shock Analog-Digital GA-2100-7AJF ($90)
At this price range it is hard to avoid Casio and their G-Shock range of watches as you can constantly find good deals on these watches. This watch in particular you get both an analog and digital dual combo on the dial as well as a huge array of functions. It features a perpetual calendar which will show you the day, date and month and includes leap years until the year 2100, a chronograph, countdown timer, alarm and a backlight to illuminate the dial in the dark. It comes in a case size of 45.4mm and is made out of polymer carbon, it is also water resistant to 200m.
Timex Harborside ($75)
If you are looking for an affordable diver this is probably as cheap as you can get while also getting a great deal with a well-respected brand. The Harborside from Timex does feature a diver bezel however, it does not rotate so keep that in mind if you do want a unidirectional bezel. It comes in a case size of 42mm and comes in a beautiful blue dial with contrasting gold indices as well as hour and hands. It is paired with a brown leather strap and unfortunately is only water resistant to 50m. While it might not serve the functions necessary to be a diver, it does qualify in everything else aesthetically.
Casio A158WA-1JF ($26)
It was hard to pick another watch at this price range without avoiding adding Casio again, but when it comes to extremely cheap and reliable watches, they always seem to be the first option. This watch is no different and is loved and adored by most watch collectors for its simplicity and its incredible reliability and functionality. The case size is 33.2mm and only 8.2mm thick with an LCD display. It features a chronograph, alarm, perpetual calendar backlight and a date and day display that can be cycled through by pressing the buttons on the side of the watch.
Seiko Alba Riki Watanabe AKQK463 ($99)
A ladies watch that could rival a lot of other its competitors purely for the price point. You get an elegant white dial with black hour and minute hands in a stainless-steel case at a size of 28mm. This watch can be purchased in three different colour leather bands, green, red and brown. While all three styles come at the same price point, one thing to note is that the brown leather strap version comes with a beige dial instead of a white dial.
Fossil Scarlette Micro Three-Hand Date Stainless Steel Watch ($70)
If the previous dial seemed a bit boring, you can always go for the Fossil Scarlette lady’s watch. This particular model features a blue mother of pearl dial and features cubic zirconia around the bezel as well as on the hour markers. Just like the Seiko Alba it comes in a case size of 28mm, but features a stainless-steel bracelet. It also features a cyclops date design, further elevating its aesthetic to be a bit flashier. There are a lot of different colour options available from the dial to the bracelet, and a good deal can be found from many websites including Fossil themselves.