The complexity of the app has a direct bearing on how much it will cost and how long it will take to develop it. There is really no such thing as a "standard" custom mobile app, but it is possible to estimate time and price based on certain parameters.
There are several online calculators available that allow you to enter the specifics of your project to obtain an estimate of the related costs. Some of them also allow you to estimate your cost of ownership, or how much it will likely cost you to support, upgrade and modify the app in the future.
This is one area that is often overlooked. As technology changes, your app will need to change with the times. Currently, mobile devices include smartphones, laptops, phablets and tablets. As the public begins to embrace wearable tech, apps will need to be optimized for an increasing number of devices.
It may be that your only solution is to have a custom mobile app developed. Perhaps the nature or scope of your business requires unusual or unique features. However, in most instances, an app with at least some of the features you need has already been developed. With a little research, you can probably find an off-the-shelf solution that will satisfy part of your requirements, and frequently, the cost of adding the additional functionality you need will be much less than a custom app.
Mobile enterprise application platforms (MEAPs) are available from a number of vendors. MEAPs are middleware-server platforms designed to deliver mobile app solutions that allow you to choose the level of programming involved. Costs can be difficult to determine, but server-licensing fees can run $50,000 to $60,000 per year for an unlimited number of users, but if you only need to serve 100 or so mobile users, the costs are roughly half that amount.
Another alternative that is typically less expensive is software-as-a-service, or SaaS. As an example, Salesforce can provide a mobile CRM solution for a monthly fee of approximately $125 per user. You can also choose a Salesforce solution for custom mobile apps for around $80 per month per user for as many as 10 different apps.
A third potential solution is a relatively new concept. Services such as Formatus allow customers to purchase units of "license months" that they can use as they are needed. To illustrate, a customer might purchase 100 license months at a total cost of around $3,500. They could spend all of their months to license 100 users for one month, 20 users for five months or one user for 100 months. Customers pay an additional fee to use the Formatus tools to create custom apps, but in most cases, this fee is far less than the cost of hiring a developer to create a custom app.
In the end, only you can decide whether to have a custom mobile app developed. You have to evaluate your specific needs, your budget, what you stand to gain from a custom app and calculate your return on investment. In the long run, a custom app may be your best solution, but you owe it to yourself to evaluate all options before making a commitment.