Engagement Party: Good news, this is traditionally thrown by the parents or close relatives. Better news, traditionally you are not required to buy a gift. If you decide to bring a gift, you should only free required to spend around $25. Maybe a nice bottle of wine?
Bride Tip: If you don't register till after your engagement party that shows your guest that you really don't expect a gift.
Bridal Shower Party and Gift: Traditionally this is thrown and organized by the Maid of Honor with help from the Bridesmaids. This party and cost anywhere from $500 and upward. And don't forget to bring a gift (anywhere between $25-$75).
Cost Cutting: Try having the party at a house. Have a brunch rather than a dinner. How about just sweets and teas?
Bachelorette Party: This also falls to the Maid of Honor with help from the Bridesmaids. That does not mean you are required to pay for everyone's drinks, but it does mean you are in charge of organizing it. Cost can range anywhere from $100 up depending on what you are doing.
Cost Cutting: Try planning the weekend (or evening) to a destination in which housing can be provided. Or better yet try a staycation!