You don't exactly need to wear any particular biking shorts just to do short rides like this. The extra padding is not going to make much of a difference if you are new to riding. Wear it or not.. your butt will ache heheh.
50++ for a padded crotch shorts is almost hard to find. Most are about $79 on average with the padded and lycra material. You want to use those MTB shorts with built in padded shorts will set you back about $100+
A pair of sport shorts or any kind of active sports shorts will be fine.
Just to let you know, for seats.. the more soft and cushy it gets.. it is NOT good. In fact they do more damage to you over time and casue more chaffing to your thigh and groin. Alot of season cyclist we use pretty hard seats. Some use carbon fibre seat which are entirely solid. And the only padding is from the cycling shorts we wear. But yet we can ride long distance without problem.