Welcome to the forum!
More expensive bikes are usually lighter, have better components, are more durable, and you can ride faster on them without worrying about the bike falling apart underneath you.
Something cheap is good if you are just starting out in cycling, but if you plan to ride with a group or if you really get into cycling, you will probably need a better bike, but right now it seems to me that you are just trying to get fitter and that bike is perfect for your level.