Universal design is a great way to age-in-place and stay in your home for life. The best way to remodel your home for aging is long before you need your home to be accessible.
Statistics show that most people would do almost anything reasonable to avoid moving from the comfort of their home yet in many cases their homes are not adapted to accommodate their current and changing needs. The question arises as to what is necessary and the associated costs to make the modifications to enable an individual to continue living safely and as independently as possible in their homes.
The answer depends on what and how much is done and each case is different. Allow me to address some typical adaptations and their associated costs in my part of the US.
The very basic modifications may involve additional hand railings, grab bars & improved lighting which may cost in the $300 to $1500 range based on the extent of work being done. Many times designer grab bars can be installed that look like stylish towel racks.
Considerations for entering and leaving the home or accessing a second level may be part of the evaluation. Simple options for entry and exit can be obtained by trip-free thresholds, access ramps & improved hardware which might cost in the range of $1500 to $3500 once again based on the extend of the work.
An automatic door opener that converts a regular door into an automatic door at the push of a button can add an additional $1300-$2000 based on availability of electricity and type of automatic opener selected.
More involved options such as stair chairs, platform lifts and even elevators can cost as little as $4000 and as much as $70,000 depending on the equipment selected.
Bathroom modifications can be as simple as installing comfort height toilets with well placed grab bars, lever type or motion activated faucets and knee space at a sink. Costs for these modifications may range $1000-$3000.
Bathrooms however tend to be the most important consideration both for the dignity and independence of the user as well as for prevention of injuries to caregivers and loved one’s taking care of the person. Entering and leaving the bathing zone becomes a major consideration.
In this case a roll in shower with no curb that allow for easy walker or wheelchair access are great additions. There are several manufacturers offering multi-piece units that work well in a remodel and when complete eliminate the obstacles encountered when entering or leaving a curbed shower or stepping over a bathtub. A typical cost for supply and installation of this type of unit will be $7-$10,000. An option also exists to create a handicapped shower with tile or stone which while offering an upgraded look is considerably more time consuming and costly. One can expect to spend in the range of $15-$25,000 based on the level of finishes selected and the size of the project.
Although if you prefer baths, a bath lift may be a better answer than a roll in shower. Bath lifts are significantly less expensive around $500.00. Bath lifts allow you to sit on the lift at the edge of the tub, put your legs into the tub than the lift lowers you down to the bottom of the tub for your bath.
For those who still prefer to soak in a bathtub the popular walk in bathtubs may be a great addition. 
These bathtubs include an in-swing door & only require stepping over a 5-7” curb. They allow the user to soak in warm water and utilize the optional air jets for possible pain reduction therapy as a result of the oxygenation of the water. I do NOT recommend these tubs, then tend to leak and the manufacturers rarely will service their leaky tubs.
For those unable to step over a curb a handicapped bathtub may be the answer. These units include an out-swing door allowing for either a lateral transfer from a wheelchair or where necessary manual assistance from a caregiver. Costs to supply and install both these types of units range from $10-$20,000.
Finally, hire an occupational therapist to assist in a design that will work for life and will provide all the recommendations and solutions available to the specific individual which may also include a handicap bathroom design should this be necessary. Consultations typically span the $300-$500 range although often times health insurance and medicaid will pay for an occupational therapy evaluation. If plans are necessary an additional $2000-$5000 is likely based on how much work is being undertaken.