The cost of participating in a triathlon can vary.
Race entries can range from $5 per race for a club run event up to $35+ for a race being delivered by an external provider. These costs vary depending on whether the race is being delivered by volunteers or professional events companies, whether roads are being closed for the event, the scale and size of the event etc. Race entries are less for members as they don't have to pay for a one day race license for insurance purposes.
To participate in triathlon you don't need a great deal of equipment. As long as you have a bike that is safe (i.e. brakes work), a helmet and some running shoes you can participate!